Systems and methods for reducing polarization in imaging detectors

ABSTRACT

A method is provided including acquiring detection events with a radiation detector including a semiconductor plate and configured to produce electrical signals in response to absorption of ionizing radiation in the semiconductor plate, wherein electrons and holes are generated responsive to absorption of the ionizing radiation. The semiconductor plate includes a first surface opposed to a second surface, with sidewalls interposed between the first surface and the second surface. A cathode electrode is disposed on the first surface and pixelated anode electrodes are disposed on the second surface. The method also includes optically coupling infrared (IR) radiation into a first portion of at least one of the sidewalls of the semiconductor plate of the radiation detector, and not coupling IR radiation into a second portion of the at least one of the sidewalls.

RELATED APPLICATIONS

The present application is a continuation-in-part of, and claimspriority to, U.S. patent application Ser. No. 14/994,895, filed Jan. 13,2016, entitled, “Systems and Methods for Reducing Polarization inImaging Detectors,” the subject matter of which is hereby incorporatedin its entirety.

BACKGROUND

The subject matter disclosed herein relates generally to imagingsystems, and more particularly to reducing polarization in imagingdetectors.

In medical imaging, ionizing radiation detectors (e.g., detectorsconfigured for detection of X-rays and/or Gamma-rays) may be used, forexample, for high flux photon counting and medical imaging, which mayalso be referred to as molecular imaging (MI). For example, ionizingradiation detectors may be used for one or more of spectral-computedtomography (Spectral-CT or Color-CT, which is capable of countingphotons and measuring their energy), single photon emission computedtomography (SPECT) in combination with CT (SPECT-CT), positron emissiontomography (PET) combined with CT (PET-CT), or magnetic resonanceimaging (MRI) combined with CT (MRI-CT).

In ionizing radiation detectors (e.g., CdZnTe (CZT) detectors) for highflux photon counting, such as used for example in Spectral-CT orColor-CT, a polarization effect may be produced by the formation ofpositive space charge in a detector. The space charge may reduce theinternal electrical field in the detector, which may degrade performanceof the detector. Degradation may also result by the attracting, by thepositive space charge, of electrons toward a cathode, thereby reducingthe drift of the electrons that should occur under ideal operationtoward the anode via the detector bias (or internal field). Underrelatively high flux of ionizing radiation, such as X-rays orGamma-rays, a strong positive space-charge may be formed, which maycause the internal electrical field in the detector to collapse,resulting in cessation of operation of the detector.

When a detector is irradiated by relatively high ionizing radiationflux, the formation of a positive space charge may be mainly created bytwo causes. First, a positive space charge may be created due toionization of long lifetime deep level hole traps. Second, a positivespace charge may be created due to low or reduced mobility of holes thatare outside of a hole trap. For example, each ionizing photon absorbedin the detector may create an electron-cloud and a holes-cloud. Underirradiation of a relatively high flux of ionizing photon, many electronsand hole clouds are formed in the detector by a large amount of ionizingphotons absorbed in the detector. Due to low mobility of the holeclouds, the clouds are not collected by the cathode and instead remainin the detector bulk after most of the electrons clouds have beencollected by the anodes. The hole clouds left in the detector bulkcreate a large positive space charge in the detector, which reduces theelectrical field in the detector and attracts electrons toward thecathode, thereby degrading detector performance. While the use ofrelatively thin detectors with blocking cathodes operated at high biasvoltage may improve hole drift velocity and detector performance whilemaintaining relatively low leakage current, further improvement is stilldesired, and such relatively thin detectors may be unsuited for use withNuclear Imaging and X-ray applications due to their relatively lowstopping power.

BRIEF DESCRIPTION

In accordance with an embodiment, a method is provided that includesacquiring detection events with a radiation detector. The radiationdetector includes a semiconductor plate. The detector is configured toproduce electrical signals in response to absorption of ionizingradiation in the semiconductor plate, wherein electrons and holes aregenerated responsive to absorption of the ionizing radiation. The holesinclude groups of holes having different effective masses forcorresponding different valence energy bands. The semiconductor platecomprises a first surface, a second surface, and sidewalls. The firstsurface is opposed to the second surface and the sidewalls areinterposed between the first surface and the second surface. A cathodeelectrode is disposed on the first surface and pixelated anodeelectrodes are disposed on the second surface. The method also includesoptically coupling infrared (IR) radiation into a first portion of atleast one of the sidewalls of the semiconductor plate of the radiationdetector, and not coupling IR radiation into a second portion of the atleast one of the sidewalls. The IR radiation has at least one wavelengthselected from a spectral range including wavelengths to which thesemiconductor plate is partially transparent and which are configured toexcite at least some of the holes from a first group at a first valenceenergy band to a second group at a second valence energy band, whereinthe holes of the second group have lower effective masses thancorresponding holes of the first group.

In accordance with another embodiment, a radiation detector is provided.The radiation detector includes a semiconductor plate, an infrared (IR)radiation source, and at least one processor. The detector is configuredto produce electrical signals in response to absorption of ionizingradiation in the semiconductor plate, wherein electrons and holes aregenerated responsive to absorption of the ionizing radiation. The holesinclude groups of holes having different effective masses forcorresponding different valence energy bands. The semiconductor platecomprises a first surface, a second surface, sidewalls, a blockingcathode electrode having a non-symmetrical current response, andpixelated anode electrodes. The first surface is opposed to the secondsurface and the sidewalls are interposed between the first surface andthe second surface. The blocking cathode electrode is disposed on thefirst surface and the pixelated anode electrodes disposed on the secondsurface. The IR radiation source is configured to provide IR radiationto the semiconductor plate via at least one sidewall. The at least oneprocessor is operably coupled to the semiconductor plate and configuredto provide IR radiation into the semiconductor plate from the IRradiation source. The IR radiation has at least one wavelength selectedfrom a spectral range including wavelengths to which the semiconductorplate is partially transparent and which are configured to excite atleast some of the holes from a first group at a first valence energyband to a second group at a second valence energy band, wherein theholes of the second group have lower effective masses than correspondingholes of the first group.

In accordance with another embodiment, a tangible and non-transitorycomputer readable medium is provided that includes one or more softwaremodules configured to direct one or more processors to acquire detectionevents via a radiation detector that includes a semiconductor plate andis configured to produce electrical signals in response to absorption ofionizing radiation in the semiconductor plate, wherein electrons andholes are generated responsive to absorption of the ionizing radiation.The holes include groups of holes having different effective masses forcorresponding different valence energy bands. The semiconductor platecomprises a first surface, a second surface, and sidewalls. The firstsurface is opposed to the second surface and the sidewalls areinterposed between the first surface and the second surface. A blockingcathode electrode is disposed on the first surface and pixelated anodeelectrodes disposed on the second surface. The one or more softwaremodules are also configured to direct the one or more processors tooptically couple infrared (IR) radiation into a first portion of atleast one of the sidewalls of the semiconductor plate of the radiationdetector, and not couple IR radiation into a second portion of the atleast one of the sidewalls. The IR radiation has at least one wavelengthselected from a spectral range including wavelengths to which thesemiconductor plate is partially transparent and which are configured toexcite at least some of the holes from a first group at a first valenceenergy band to a second group at a second valence energy band, whereinthe holes of the second group have lower effective masses thancorresponding holes of the first group.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic view of a radiation detector assemblyaccording to an embodiment.

FIG. 2 illustrates an example of energies of various bands as a functionof electron momentum.

FIG. 3 shows a flowchart of a method, according to an embodiment.

FIG. 4 shows a schematic view of an imaging system, according to anembodiment.

FIG. 5 schematically illustrates the energy band structures E_(n) for asemiconductor detector as a function of the coordinate Z.

FIG. 6 schematically illustrates the electrical field E for asemiconductor detector as a function of the coordinate Z.

FIG. 7 schematically illustrates the number of absorbed photons N for asemiconductor detector as a function of the coordinate Z.

FIG. 8 illustrates a graph including a curve representing a calculatednormalized induced charge on an anode 114 as a function of theDepth-Of-Interaction (DOI) in a non-depletion region.

FIG. 9 shows a schematic view of a spectrum including a low energy tail.

FIG. 10 shows a flowchart of a method, according to an embodiment.

FIG. 11 provides a schematic view of a radiation detector assemblyaccording to an embodiment.

DETAILED DESCRIPTION

The foregoing summary, as well as the following detailed description ofcertain embodiments and claims, will be better understood when read inconjunction with the appended drawings. To the extent that the figuresillustrate diagrams of the functional blocks of various embodiments, thefunctional blocks are not necessarily indicative of the division betweenhardware circuitry. Thus, for example, one or more of the functionalblocks (e.g., processors, controllers or memories) may be implemented ina single piece of hardware (e.g., a general purpose signal processor orrandom access memory, hard disk, or the like) or multiple pieces ofhardware. Similarly, the programs may be stand alone programs, may beincorporated as subroutines in an operating system, may be functions inan installed software package, and the like. It should be understoodthat the various embodiments are not limited to the arrangements andinstrumentality shown in the drawings.

As used herein, the terms “system,” “unit,” or “module” may include ahardware and/or software system that operates to perform one or morefunctions. For example, a module, unit, or system may include a computerprocessor, controller, or other logic-based device that performsoperations based on instructions stored on a tangible and non-transitorycomputer readable storage medium, such as a computer memory.Alternatively, a module, unit, or system may include a hard-wired devicethat performs operations based on hard-wired logic of the device.Various modules or units shown in the attached figures may represent thehardware that operates based on software or hardwired instructions, thesoftware that directs hardware to perform the operations, or acombination thereof.

“Systems,” “units,” or “modules” may include or represent hardware andassociated instructions (e.g., software stored on a tangible andnon-transitory computer readable storage medium, such as a computer harddrive, ROM, RAM, or the like) that perform one or more operationsdescribed herein. The hardware may include electronic circuits thatinclude and/or are connected to one or more logic-based devices, such asmicroprocessors, processors, controllers, or the like. These devices maybe off-the-shelf devices that are appropriately programmed or instructedto perform operations described herein from the instructions describedabove. Additionally or alternatively, one or more of these devices maybe hard-wired with logic circuits to perform these operations.

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” are not intended to beinterpreted as excluding the existence of additional embodiments thatalso incorporate the recited features. Moreover, unless explicitlystated to the contrary, embodiments “comprising” or “having” an elementor a plurality of elements having a particular property may includeadditional such elements not having that property.

As noted above, positive charge spaces may be created by the generationof hole traps having non-mobile holes, as well as reduced mobility ofholes that are not in traps. For example, holes in a valence band mayhave varying effective masses, with the holes with higher effectivemasses having lower mobility and thus reducing detector performance.Thus, in contrast to previous approaches that may have only addressedholes in traps, various embodiments provide systems and methods thatreduce polarization by increasing the mobility of holes in a valenceband. In some embodiments, the number of holes in traps is also reducedto further improve detector performance. Various embodiments providesystems and methods that utilize a radiation detector (e.g., a CZTradiation detector) for high flux photon counting (e.g., Spectral-CT)with reduced polarization effects. Various embodiments provide systemsand methods that utilize the same detector for Nuclear Imaging (NI) aswell as MI or Spectral-CT, for example.

In various embodiments, the radiation detector may be thick enough toprovide the stopping power needed in MI applications but still provideimproved holes drift-velocity for reduced polarization. For maintainingthe same drift time for the charge-carriers in the detector for variousthicknesses, the high voltage bias of the detector may increase as thesquare of the detector thickness d, and the leakage-current may increaseproportionally to the detector thickness. Accordingly, for maintaininghigh drift-velocity for the holes charge-carriers in the detector, asneeded for high flux photon counting, and at the same time maintainingthe detector as thick enough as needed for MI application, the leakagecurrent in the detector may tend to become high enough to result in poorenergy resolution. Accordingly, various embodiments provide for theconversion of holes having heavy effective-mass into lightereffective-mass to allow higher drift-velocity for the holes even inthick detectors (having lower internal electrical field) that will bebeneficial for both, high flux photon counting, such as CT applications,and MI applications such as SPECT.

In various embodiments, in order to reduce the formation of the positivespace-charge in the radiation detector, the holes' drift-velocity may beincreased, by increasing the mobility of the holes. Increasing the holesmobility and drift-velocity allows the holes to be collected by thedetector cathode during or shortly after the time in which most of theelectron clouds have been collected by the detector anode. This ensuresthat most of the hole clouds will not be left in the detector bulk afterthe collection of the electron clouds, thereby preventing or reducingthe formation of positive space-charge and the polarization effect.

Various embodiments increase the holes' mobility and drift-velocity byconverting Heavy Holes (HH) into Light-Holes (LH) and Spin-Orbit orSplit-Off Holes (SOH) for reducing the effective-mass of the holes,thereby increasing their mobility. The conversion of HH into LH may beproduced by optical excitation of Near, Mid and/or Far Infra-Red (IR)radiation that excites electrons from the LH-band into the HH-band andfrom the spin-orbit band into the HH and LH bands. Such excitation ofelectrons is equivalent to the excitation or transition of holes fromthe HH-band into the LH-band and from the HH and LH bands into thespin-orbit band (SO) band. As used herein, the effective mass of aparticle may be understood as the mass the particle seems to have whenresponding to forces, or the mass the particle seems to have whengrouped with other identical particles. The effective mass of anelectron or electron hole may be stated in units of true mass of theelectron.

In the above mentioned process of converting HH into LH or SOH, theexcitation of the electrons includes their transition from occupiedstates in the LH-band into unoccupied states in the HH-band, which mayhave been produced by the excitation of electrons from the HH-band intothe conduction band by X-Rays or Gamma-Rays radiation. The unoccupiedstates of electrons in the HH band may also be produced by P-type dopingof the semiconductor from which the radiation detector is made. In adegenerated state of P-type semiconductor the quasi Fermi-level E_(f) islocated inside the valence energy-band (or valence energy-bands) andthus parts of these bands (above the quasi Fermi-level E_(f)) may not beoccupied by electrons and include the unoccupied electrons-states (orthe states occupied by holes).

A technical effect of at least one embodiment includes improved imagequality, especially at lower doses (e.g., due to correcting oraddressing polarization). A technical effect of at least one embodimentincludes reduced polarization. A technical effect of at least oneembodiment includes improved flexibility allowing a radiation detectorto be advantageously used with multiple different modalities. Atechnical effect of at least one embodiment includes better contrast,lower dose, and/or reduction or elimination of “after-glow” artifacts.

FIG. 1 provides a schematic view of a radiation detector assembly 100 inaccordance with various embodiments. As seen in FIG. 1, the radiationdetector assembly 100 includes a semiconductor plate 110, a processingunit 120, and an infrared (IR) radiation source 130. Generally, thesemiconductor plate 110 is configured to produce electrical signals inresponse to absorption of ionizing radiation in the semiconductor plate110. Electrons and holes are generated responsive to absorption of theionizing radiation. The holes include groups of holes having differenteffective masses for corresponding different valence energy bands. Forexample, a first group of holes may have a first effective mass (orrange of effective masses) for a corresponding first valence energyband. A second group of holes may have a second effective masse (orrange of effective masses) that is different from the first effectivemass for a corresponding second valence energy band that is differentfrom the first valence energy band. The IR radiation source 130 isconfigured to provide IR radiation to semiconductor plate 110. Theprocessing unit 120 is operably coupled to the semiconductor plate 110,and is configured to provide IR radiation into the semiconductor plate110 from the IR radiation source 130. The IR radiation may be utilizedto convert holes from one valance band to another during the receptionof ionizing radiation from an object being imaged. The IR radiationincludes at least one wavelength selected (e.g., determined by theprocessing unit 120) from a spectral range including wavelengths towhich the semiconductor plate 110 is partially transparent and which areconfigured to excite at least some of the holes (e.g., holes generatedby the absorption of ionizing radiation by the semiconductor plate 110)from a first group at a first valence energy band to a second group ofholes a second valence energy band. The holes of the second group havelower effective masses than corresponding holes of the first group.Thus, by converting the holes from the first group to the second group,hole mobility is increased and polarization is decreased.

The depicted semiconductor plate 110 includes a first surface 111, asecond surface 113, sidewalls 116, a monolithic cathode electrode 112,and pixelated anode electrodes 114. In various embodiments, pluralcathode electrodes may be employed instead of a monolithic cathodeelectrode. The first surface 111 is opposed to the second surface 113,with the sidewalls 116 interposed between the first surface 111 and thesecond surface 113 and extending therebetween. The monolithic cathodeelectrode 112 is disposed on the first surface 111, and the pixelatedanode electrodes 114 are disposed on the second surface 113. The IRradiation source 130 in various embodiments is configured to direct IRradiation into the semiconductor plate 110 via the sidewalls 116.Additionally or alternatively, the IR radiation source 130 may beconfigured to direct IR radiation into the semiconductor plate 110 viathe monolithic cathode electrode 112. For example, the monolithiccathode electrode 112 may be at least partially transparent to the IRradiation provided by the IR radiation source 130. Additional discussionregarding the direction of IR radiation into a semiconductor plate ordetector may be found in U.S. Pat. No. 7,800,071, issued Sep. 21, 2010and entitled “Method, Apparatus, and System of Reducing Polarization inRadiation Detectors,” the subject matter of which is incorporated hereinin its entirety. The semiconductor plate 110 in various embodiments maybe constructed using different materials, such as semiconductormaterials, including Cadmium Zinc Telluride (CdZnTe), often referred toas CZT, Cadmium Telluride (CdTe), or other semiconductors having splitvalence energy bands. Generally, when radiation (e.g., one or morephotons) impacts the pixelated anode electrodes 114, the semiconductorplate 110 generates electrical signals corresponding to the radiationbeing absorbed in the volume of the semiconductor plate 110. In theillustrated embodiment, the pixelated anode electrodes 114 are shown ina 3×3 array for a total of 9 pixelated anode electrodes 114; however, itmay be noted that other numbers or arrangements of pixelated anodes maybe used in various embodiments. Each pixelated anode electrode 114, forexample, may have a surface area of 2.5 millimeters square; however,other sizes and/or shapes may be employed in various embodiments.

It may be noted that each pixelated anode electrode 114 may haveassociated therewith one or more electronics channels configured toprovide signals to one or more aspects of the processing unit 120 incooperation with the pixelated anodes. In some embodiments, all or aportion of each electronics channel may be disposed on the semiconductorplate 110. Alternatively or additionally, all or a portion of eachelectronics channel may be housed externally to the semiconductor plate110, for example as part of the processing unit 120, which may be orinclude an Application Specific Integration Circuit (ASIC).

As discussed above, the IR radiation source 130 is configured to provideIR radiation to the semiconductor plate 110. In the depicted embodiment,the IR radiation source 130 includes a light source 132 and light guides134 coupled to the light source 132. The light guides 134 are interposedbetween the light source 132 and the semiconductor plate 110 and guidelight from the light source 132 to the semiconductor plate 110. Thelight guides 134, in various embodiments, may include one or more Bragggrating. In various embodiments, the IR radiation source 130 mayinclude, for example, a CO₂ laser having two of its emission lines at9.4 μm and 10.6 μm wavelengths.

As seen in FIG. 1, the depicted processing unit 120 is operably coupledto the semiconductor plate 110 (e.g., the pixelated anode electrodes 114and/or associated electronic channels or processing circuitry), as wellas the IR radiation source 130. The processing unit 120 is configuredto, among other things, determine or otherwise select a wavelength of IRradiation to provide to the semiconductor plate 110 to convert holesgenerated by absorbed ionizing radiation from an object to be imagedfrom at least one valence band to at least one other valence band havinglower effective mass (and improved mobility), and to control the IRradiation source 130 to provide the IR radiation at the determinedwavelength (or wavelengths).

For example, as also discussed in connection with FIG. 2 below, in someembodiments the different valence energy bands may include a heavy-holeband corresponding to heavy-holes, a light-hole band corresponding tolight-holes, and a spin-orbit band corresponding to spin-orbit-holes.The spin-orbit holes have lower effective masses than the light-holes,and the light-holes have lower effective masses than the heavy-holes.The processing unit 120 and the IR radiation source 130 are configuredin various embodiments to provide the IR radiation to the semiconductorplate 110 to cause transition of at least one of: (1) at least someholes from a group corresponding to the heavy-hole band into holes of agroup corresponding to the light-hole band; (2) at least some holes froma group corresponding to the heavy-hole band into holes of a groupcorresponding to the spin-orbit band; or (3) at least some holes fromthe group corresponding to the light-hole band into holes of the groupcorresponding to the spin-orbit band.

In various embodiments the processing unit 120 includes processingcircuitry configured to perform one or more tasks, functions, or stepsdiscussed herein. It may be noted that “processing unit” as used hereinis not intended to necessarily be limited to a single processor orcomputer. For example, the processing unit 120 may include multipleprocessors, ASIC's and/or computers, which may be integrated in a commonhousing or unit, or which may distributed among various units orhousings. It may be noted that operations performed by the processingunit 120 (e.g., operations corresponding to process flows or methodsdiscussed herein, or aspects thereof) may be sufficiently complex thatthe operations may not be performed by a human being within a reasonabletime period.

In the illustrated embodiment, the processing unit 120 includes acontrol module 122 and a memory 124. It may be noted that other types,numbers, or combinations of modules may be employed in alternateembodiments, and/or various aspects of modules described herein may beutilized in connection with different modules additionally oralternatively. For example, the processing unit 120 may include one ormore modules configured to acquire data from the semiconductor plate 110and to reconstruct one or more images using the acquired data.Generally, the various aspects of the processing unit 120 actindividually or cooperatively with other aspects to perform one or moreaspects of the methods, steps, or processes discussed herein.

In the illustrated embodiment, the depicted control module 122 isconfigured to determine one or more wavelengths of IR radiation toprovide to the semiconductor plate 110 for improving the mobility ofholes within a valence band. The control module 122 is also configuredto control the IR radiation source 130 to provide the IR radiation tothe semiconductor plate 110.

The memory 124 may include one or more computer readable storage media.Further, the process flows and/or flowcharts discussed herein (oraspects thereof) may represent one or more sets of instructions that arestored in the memory 124 for direction of operations of the radiationdetection assembly 100.

FIG. 2 illustrates an example energy band structure 10. FIG. 2schematically illustrates the electrons or holes energy-bands diagram ofa semiconductor showing different possible transitions of holes betweenthe split valence energy-bands (valence bands) of a semiconductor. Itmay be noted that the energy (electronic) band structure of CdTe andCdZnTe semiconductors is similar to the one of other semiconductorshaving Zinc blended crystals structures, such as Germanium. FIG. 2schematically shows the electrons or holes energy-band structure 10 of aCdTe or CdZnTe (CZT) semiconductor, or other semiconductors having Zincblended crystal structures. Energy band structure 10 depicted in FIG. 2shows the energy E (12) of the bands as a function of the electronmomentum K (14). The energy band structure 10 includes the conductionband 16, Heavy Holes (HH) valence band 18, Light Holes (LH) valence band20 and Split-Off or Spin-Orbit (SO) valence band 22. In the specificexample depicted in FIG. 2, the HH and LH bands 18 and 20, respectivelyare degenerate at momentum K=0 and have a common point there. In CdTe orCZT detectors, the SO band 22 may be separated from the HH band 18 atthe vicinity of an electron momentum of K=0 by energy that is largerthan the energy difference between the HH and the LH bands 18 and 20,respectively. The band structure of FIG. 2 depicts a situation when theCZT is a degenerate P-type semiconductor and thus the quasi Fermi-LevelE_(f) (32) is inside the valence HH and LH bands.

As shown in FIG. 2, dotted area or region 30 schematically depicts theregion that is fully occupied by electrons. Region 30, in its upperenergy limit, reaches the quasi Fermi-Level E_(f) (32). Above quasiFermi-Level E_(f) (32), the electrons or holes split valence bands areunoccupied by electrons (or occupied by holes). Thus, in the specificexample shown by FIG. 2, HH and LH bands 18 and 20, respectively, areunoccupied by electrons (and are instead occupied by HH and LH,respectively) in the electron momentum range K between K=K₁ and K=K₂. Itmay be noted that in intrinsic or slightly N-type semiconductors, thequasi Fermi-Level E_(f) (32) is located near the mid energy-gap E_(G)(42) between the HH band 18 and the conduction bands 16. In such asituation, none of the electrons or holes split valence bands 18, 20 and22 are occupied with holes. However, even under the conditions mentionedabove (i.e. intrinsic or slightly N-type semiconductor), HH band 18 maynevertheless include a portion with unoccupied states of electrons (oroccupied states of HH) when the ionizing radiation has intensity that ishigh enough to excite enough electrons from HH band 18 to conductionband 16 for creating a quasi-equilibrium state in which HH band 18 isoccupied by HH holes. Accordingly, for example under high flux ofionizing radiation, HH band 18 may be occupied by HH even when thesemiconductor is an intrinsic semiconductor or even slightly N-typesemiconductor. Accordingly, various embodiments may be employed fordegenerate P-type, P-type, intrinsic, and/or slightly N-typesemiconductors. It may be noted that the momentum range between K=K₁ andK=K₂ in which HH band 18 is occupied by HH may be determined, forexample, based on the amount of doping of the semiconductor, thetemperature of the semiconductor that determines the position of quasiFermi-Level E_(f) (32), and/or the intensity of the flux of the ionizingradiation.

As seen in FIG. 2, there are three possible transitions between thesplit bands of the valence band that may result in improved holemobility. As depicted in FIG. 2, these three possible transitions are:(1) Transition 24, type I: transition of HH from the HH band 18 to theLH band 20 (HH→LH); (2) Transition 26, type II: transition of LH fromthe LH band 20 to the SO band 22 (LH→SO); (3) Transition 28, type III:transition of HH from the HH band 18 to the SO band 22 (HH→SO).

The following discussion below focus on transition 24, type I (HH→LHtransition); however, it should be understood that the generalprinciples of the discussion regarding transition 24 generally appliesto the other holes transitions, mentioned above, as well. The lifetimeof holes excited from the HH band into the LH band is in the Pico-second(PS) range. Accordingly, the relaxation of holes from the LH-band backto the HH-band is almost unaffected by the recombination of electronsfrom the conductance-band to the LH-band since such transitions have alife time of 1 μs, which is much longer than the PS lifetime of theholes in the LH-band. In addition the direct transition from theconductance-band to the LH-band has very low probability.

According to the simplified electron energy band, the mathematicalrelations between the electron energy E(K) and its momentum K are givenby:

$\begin{matrix}{{E(K)} = {\frac{h^{\backprime 2}}{2\; m^{*}} \cdot K^{2}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$where E(K) is the electron energy in the energy band, h′ is the reducedPlank constant h, h′=h/2π, m* is the effective-mass, and K is theelectron momentum. Taking the second derivative of Eq. (1) andrearranging the equation gives the mathematical expression for theeffective mass m*:

$\begin{matrix}{m^{*} = {h^{\backprime 2}/\frac{d^{2}{E(K)}}{{dK}^{2}}}} & {{Eq}.\mspace{14mu}(2)}\end{matrix}$

From Eq. (2) and FIG. 2, it can be seen that, around momentum K=0, HHband 18, which is the shallowest, has the lowest second derivative

$\frac{d^{2}{E(K)}}{{dK}^{2}}$and thus has the highest effective-mass m*(HH). On the other hand, SOband 22, which is the steepest valence band in the vicinity of theelectron momentum K=0, has the lowest effective-mass m*(SO). Theeffective mass of LH band 20 m*(LH) satisfies the mathematical relation:m*(SO)<m*(LH)<m*(HH)

It may be noted that, in A. Rubi-Ponce et. al. (“Calculation ofEffective Masses of II-IV Semiconductor Compounds”, Superficies Vacio16(2), 26-28, Junio de 2003), the following values for the differenteffective-masses are reported for semiconductors having Zinc blendedcrystal structures: m*(HH) of 0.892, m*(LH) of 0.112, and m*(SO) of0.508, where the values of effective-mass are given in units of electronfree mass.

It may be seen that m*(LH) is about 8 times lighter than m*(HH), andthat m*(SO) is about 2 times lighter than m*(HH). The value of thesemasses depends on the crystal orientation, which in the depicted case is[111], and on the value of the electrons momentum K. Accordingly, inthis specific example, and unlike the situation when the electronsmomentum K=0, m*(SO)>m*(LH). According to the specific example above,conversion of HH into LH by transition of holes from the HH band intothe LH band (HH→LH) will reduce their effective mass m*(LH) by a factorof 8 relative to their initial mass m*(H H). Accordingly, thedrift-velocity and mobility of the converted LH is about 8 times higherthan the drift-velocity and mobility of the original HH. Similarly,conversion of HH into SOH by transition of holes from the HH band intothe SO band (HH→SOH) will reduce their effective mass m*(LH) by a factorof 2 relative to their initial mass m*(HH). Accordingly, thedrift-velocity and mobility of the converted SOH is about 2 times higherthan the drift-velocity and mobility of the original HH.

It may be noted that the transition types I, II and III corresponding totransitions 24, 26 and 28 are all transitions of holes that move from avalence band with a certain effective-mass m* into another valence bandwith a smaller effective-mass m*. Accordingly, transitions 24 (HH→LH),26 (LH→SO) and 28 (HH→SO) are all transitions that convert the initialeffective-masses m*(HH), m*(LH) and m*(HH) of the holes in energy band18, 20 and 18 into smaller effective-masses m*(LH), m*(SOH) and m*(SOH),respectively. Converting the effective-mass m* of the holes into smallereffective-mass m* increases the holes' mobility and theirdrift-velocity, resulting in reduction of the positive space-charge,thereby reducing polarization effects and detector performancedegradation due to polarization.

For zinc blended crystals additional calculated information about theeffective mass m* of the holes in the split valence bands at temperatureof 300 K (as given byhttp://www.ioffe.ru/SVA/NSM/Semicond/GaN/bandstr.html?sm_au_=iMVVJ77F31nVH6kN)is: effective hole masses (heavy holes—HH) is m_(HH)=1.3 m_(o);effective hole masses (light holes—LH) is: m_(LH)=0.19 m_(o); andeffective hole masses (split-off band—SOH) is m_(SOH)=0.33 m_(o), wherem_(o) is the electron free mass.

In CZT, the mobility of the electrons is about 10 times higher than thatof the heavy holes HH. From the data above, it may be seen that theeffective mass m* of the HH may be converted into lighter effective massthat is about 6 to 8 times lighter than the effective mass of the HH.Accordingly, such conversion of HH into LH may convert the mobility ofthe HH into a mobility that is similar to the mobility of the electrons.In such a situation, when a CZT radiation detector is irradiated by highflux of ionizing radiation, little or no space charge is produced in thedetector bulk since all or most of the holes may drift fast enough to becollected by the cathode during the similar time that it takes to allthe electrons to be collected by the anodes. Accordingly, few or noholes are left in the detector bulk shortly after the collection of theelectrons is completed such that little or no space charge is produced,resulting in small or no polarization effect in the detector.

In various embodiments, conversion of HH into LH or SOH, and/or theconversion of LH into SOH may be achieved via optical excitation ofholes from the HH-band 18 into the LH or the SO bands 20 or 22,respectively, or by excitation of LH from LH band 20 into SO band 22.Such conversion may reduce the mass of the holes by a factor of 6 to 10,for example, thereby increasing the mobility of the holes. Increasingthe holes-mobility reduces the positive space-charge of holes left inthe detector bulk immediately after the electrons of the signal havealready been collected by the detector-anodes. The positive space chargeproduced in a CZT detector operating under high-flux is reduced by theimproved mobility of the holes, via the conversion of holes from HH intoLH, for example, reducing the holes effective mass m* and increasing thevelocity of the holes, thereby facilitating the holes to be collected bythe cathode.

The wavelength suitable to produce optical excitation of HH from the HHband into the LH or SO bands and from LH band into the SO band may befound, for example, experimentally by measuring the absorption of thesemiconductor from which the radiation detector is made. Such a spectrummay be capable of producing transitions of the types I, II and IIIcorresponding to transitions 24, 26 and 28, such that transitions 24(HH→LH), 26 (LH→SO) and/or 28 (HH→SO) are achieved in which holes movesfrom a valence band with a certain effective-mass m* into anothervalence band with a smaller effective-mass m*.

Next, the spectral range from which the wavelength(s) may be selectedwill be discussed. As indicated above, such a spectrum havingwavelengths corresponding to the momentum range starting at K=0 andending at K=K1, depends on the doping type, doping amount of thesemiconductor (the location of the quasi-Fermi level E_(f) (32) insidethe split valence bands, which is the degenerate amount of thesemiconductor affecting the optical absorption by the Burstein-Mossshift effect), the element composition of the semiconductor, theintensity of the ionizing radiation impinging on the radiation detector,and/or the detector temperature, for example. The penetration ofquasi-Fermi level E_(f) (32) into the HH band is measured by energyunits and is equal to ΔE_(f)(34), which may be derived by measuring theBurstein-Moss shift (the difference in the absorbed wavelengths λ₁ andλ₂ corresponding to the optical absorption, by transitions 36 and 38 ofelectrons from the HH valence band 18 to conduction band 16 under thesituations when the semiconductor is degenerate and non-degenerate,respectively). ΔE_(f) is given by:

$\begin{matrix}{{\Delta\; E_{f}} = \left( {\frac{hC}{\lambda_{1}} - \frac{hC}{\lambda_{1}}} \right)} & {{Eq}.\mspace{14mu}(4)}\end{matrix}$

Where C is the speed of light. The momentum K₁ is the value of theelectron momentum K when the energy E (K₁) of the HH band 18 (relativeto the energy at momentum K=0) is equal to the energy of quasi-Fermilevel ΔE_(f) (34) inside the HH band 18. Similarly, the momentum K₃ isthe value of the electron momentum K when the energy E(K₃) of the LHband 20 (relative to the energy at momentum K=0) is equal to the energyof quasi-Fermi level ΔE_(f) inside the LH band 20. Accordingly, from theanalogy to Eq. (1), K₁ and K₃ can be derived from Eqs. (5a)-(5d):

$\begin{matrix}{{E\left( K_{1} \right)} = {{\Delta\; E_{f}} = {\frac{h^{\backprime 2}}{2{m^{*}({HH})}} \cdot K_{1}^{2}}}} & {{Eq}.\mspace{14mu}\left( {5a} \right)} \\{Or} & \; \\{K_{1} = \sqrt{\frac{\Delta\;{E_{f} \cdot 2}{m^{*}({HH})}}{h^{\backprime 2}}}} & {{Eq}.\mspace{14mu}\left( {5b} \right)} \\{{E\left( K_{3} \right)} = {{\Delta\; E_{f}} = {\frac{h^{\backprime 2}}{2{m^{*}({LH})}} \cdot K_{3}^{2}}}} & {{Eq}.\mspace{14mu}\left( {5c} \right)} \\{Or} & \; \\{K_{1} = \sqrt{\frac{\Delta\;{E_{f} \cdot 2}{m^{*}({LH})}}{h^{\backprime 2}}}} & {{Eq}.\mspace{14mu}\left( {5d} \right)}\end{matrix}$

When K₁ and K₃ are derived, the maximum transition energies (at momentumK=K₁ and K=K₃) between the split valence bands 18, 20 and 22 may befound. The maximum transition energies ΔE(HH→LH), ΔE(LH→SO), andΔE(HH→SO) corresponding to transitions of types I, II and III ortransitions 24, 26 and 28, respectively, are given by the difference ofthe energies E(K) of the split valence bands 18, 20 and 22 at theelectrons momentum K of the transitions, such as momentums K₁ and K₃. Inthe range of these values of the momentum K, the conditions for thetransitions (HH→LH), (LH→SO), and (HH→SO) are satisfied (i.e., there arestates occupied with holes in the valance band from which holes move toanother energy band where there are vacant states of holes at thevalance band that receives the holes).

$\begin{matrix}{\mspace{79mu}{{\Delta\;{E\left( {HH}\rightarrow{LH} \right)}} = {{\frac{h^{\backprime 2}}{2{m^{*}({LH})}} \cdot K_{1}^{2}} - {\frac{h^{\backprime 2}}{2{m^{*}({HH})}} \cdot K_{1}^{2}}}}} & {{Eq}.\mspace{14mu}\left( {6a} \right)} \\{{\Delta\;{E\left( {LH}\rightarrow{SO} \right)}} = {{\Delta\;{E({SO})}} + {\frac{h^{\backprime 2}}{2{m^{*}({SO})}} \cdot K_{3}^{2}} - {\frac{h^{\backprime 2}}{2{m^{*}({LH})}} \cdot K_{3}^{2}}}} & {{Eq}.\mspace{14mu}\left( {6b} \right)} \\{{\Delta\;{E\left( {HH}\rightarrow{SO} \right)}} = {{\Delta\;{E({SO})}} + {\frac{h^{\backprime 2}}{2{m^{*}({SO})}} \cdot K_{1}^{2}} - {\frac{h^{\backprime 2}}{2{m^{*}({HH})}} \cdot K_{1}^{2}}}} & {{Eq}.\mspace{14mu}\left( {6c} \right)}\end{matrix}$

When =ΔE(SO) (40) is the split-off (spin-orbit) energy splitting betweenHH, LH bands and SO band at electrons momentum K=0. The shortesttransition wavelengths λ(HH→LH), λ(LH→SO) and λ(HH→SO) corresponding tothe maximum transition energies of Eq.(6a)-Eq.(6c) are given by:

$\begin{matrix}{{\lambda\left( {HH}\rightarrow{LH} \right)} = \frac{hC}{\Delta\;{E\left( {HH}\rightarrow{LH} \right)}}} & {{Eq}.\mspace{14mu}\left( {7a} \right)} \\{{\lambda\left( {LH}\rightarrow{SO} \right)} = \frac{hC}{\Delta\;{E\left( {LH}\rightarrow{SO} \right)}}} & {{Eq}.\mspace{14mu}\left( {7b} \right)} \\{{\lambda\left( {HH}\rightarrow{SO} \right)} = \frac{hC}{\Delta\;{E\left( {HH}\rightarrow{SO} \right)}}} & {{Eq}.\mspace{14mu}\left( {7c} \right)}\end{matrix}$

The longest transition wavelengths λ(HH→LH), (LH→SO) and λ(HH→SO)corresponding to the minimum transition energies of Eq.(6a)-Eq.(6c) in asituation when the momentum K=0 are given by:

$\begin{matrix}{{\lambda\left( {HH}\rightarrow{LH} \right)} = {{approaching}\mspace{14mu}{infinity}}} & {{Eq}.\mspace{14mu}\left( {8a} \right)} \\{{\lambda\left( {LH}\rightarrow{SO} \right)} = \frac{hC}{\Delta\;{E({SO})}}} & {{Eq}.\mspace{14mu}\left( {8b} \right)} \\{{\lambda\left( {HH}\rightarrow{SO} \right)} = \frac{hC}{\Delta\;{E({SO})}}} & {{Eq}.\mspace{14mu}\left( {8c} \right)}\end{matrix}$

Accordingly, the spectral range of wavelengths for the varioustransitions λ(HH→LH), λ(LH→SO) and λ(HH→SO) are defined by the rangebetween the shortest and longest wavelengths for each of the transitionsas expressed by the set of equations {Eq. (7a)-Eq.(7c)} and {Eq.(8a)-Eq.(8c)}, respectively.

It may be noted that while theoretically the wavelength λ(HH→LH) mayapproach infinite value, in practice there is an upper-limit to thiswavelength determined by the thermal excitation of holes from the HHband into the LH band. For small values of momentum K, the energydifference between the HH and LH bands is small, corresponding to longλ(HH→LH). In this range, where the energy difference between the HH andthe LH bands is small enough to allow efficient thermal excitation, theoptical excitation may not be particularly efficient or useful.Accordingly, the upper limit for the value of λ(HH→LH) in variousembodiments may be the value where this wavelength is less efficientthan the thermal excitation of holes between the HH and the LH bands.

Alternatively to the calculations shown above, the spectral range ofwavelengths for the various transitions λ(HH→LH), λ(LH→SO) and λ(HH→SO)may be derived experimentally using the above mentioned opticalspectral-range by measuring the optical absorption corresponding to thewavelengths range of the relevant transitions between the splitvalence-bands.

In various embodiments, a rather high-intensity optical-excitation maybe utilized at quite long wavelengths. Various embodiments may employ aCO₂ laser having two of its emission lines at 9.4 μm and 10.6 μmwavelengths. Such lasers may be commonly available, relatively low inprice, and relatively high in power.

From equation (2) above, it may be seen that the effective mass m* isproportional to

$1/{\frac{d^{\; 2}{E(K)}}{{dK}^{2}}.}$This means that the larger is the second derivative

$\frac{d^{\; 2}{E(K)}}{{dK}^{2}},$the smaller is the effective mass m*. From FIG. 2, it may be seen thatthe larger change in the energy E(k) is in the vicinity of the regionwhere the electron momentum K is close to zero, which means that thesecond derivative

$\frac{d^{\; 2}{E(K)}}{{dK}^{\; 2}}$there is larger and the effective mass m* there is the smallest. On theother hand, at larger values of electron momentum K, the HH, LH and SOsplit valence bands have generally parallel curves, which are close tolinear curves. This means that the holes in the HH, LH and SO splitvalence bands have the same small second derivatives

$\frac{d^{\; 2}{E(K)}}{{dK}^{\; 2}}$there and thus the holes in these split valence bands, in such range oflarge electrons momentum K, have similar effective mass m*. Accordingly,it may be noted that the conversion of the effective mass m* from alarge value into a smaller value, by the transitions (HH→LH), (LH→SO),and (HH→SO) may be done at relatively small electron momentum K, whichmeans optical transitions λ(HH→LH), λ(LH→SO) and λ(HH→SO) are excited byrelatively long wavelengths λ, such as the wavelength 9.4 and 10.6 μmprovided by a CO₂ laser.

It may be noted such long wavelengths are not appropriate for earlierapproaches which aimed to reduce the polarization by uniform ionizationof holes in the deep-trap levels. For example, in a CZT semiconductor,the energy gap for 10% Zn CZT is about 1.48 eV. This means that the deeplevels, which are located near the mid band-gap E_(G) (42) of the CZT,are separated from the conductance band by energy of about 0.74 eV.Accordingly, the longest wavelength for ionizing such deep trappinglevels is equal to 1.25/0.74=1.69 μm, which is far away from thewavelengths discussed herein to convert heavy holes into light holes bytransitions between the split valence bands. It may further be notedthat reducing the polarization by conversion of heavy holes into lightholes may be performed by a completely different spectral range otherthan the spectral range that may be used to produce uniform ionizationof the deep trap levels.

Accordingly, to achieve a complete or nearly complete reduction of theionization effect in a semiconductor radiation detector, such as a CZTradiation detector irradiated by high flux of ionizing radiation, twodifferent optical excitations with two different spectral ranges may beutilized. First, an optical excitation with the shorter wavelengthsrange should be used (e.g., as discussed in U.S. Pat. Nos. 7,652,258 and7,800,071, which are hereby incorporated by reference in their entirety)for producing uniform space charge by ionizing deep level hole traps.For this purpose, the longest wavelength used may be in the range of1.69 μm. Second, an optical excitation with much longer wavelengths maybe used for efficient conversion of heavy holes into light holes. Forthis purpose, the wavelength range may start with wavelengths notshorter than 3.3-5 μm for the efficient conversions of LH into SOH(LH→SO) and not shorter than 5-16.5 μm for the conversions of HH into LH(HH→LH). Accordingly, the wavelength of the optical-excitation IRradiation may be selected from a range of wavelengths in the rangebetween 3.3 μm and 16.5 μm in various embodiments.

It may be noted that conversion of HH into LH may be achieved much moreefficiently by IR and FIR (far infrared) excitation relative to heating,and, unlike heating, optical excitation may be done without increasingthe leakage-current of the radiation detector. The amount of freecharge-carriers produced in a detector, by the intensity, such as usedin diagnostic spectral CT, (10¹⁰-10¹¹ photons/cm²) of the ionizingradiation (X-Rays and Gamma Rays), and the short lifetime of the chargecarriers in the split valence bands (in the range of PS) is such thatthe free carrier absorption in the radiation detector may be considerednegligible. Accordingly, no significant heating of the detector will beproduced by the IR or FIR radiation and the conversion of HH into LH, bythe IR or FIR radiation is very efficient.

Next, the calculations of the steady-state density of HH produced by thehigh-flux X-Ray or Gamma-Rays radiation impinging on the CZT detectorwill be discussed. For example, in the case of photon counting spectraldiagnostic CT, the X-Ray radiation intensity I is 10¹⁰photons/(cm²·sec), and the steady-state density of HH produced by thisintensity I of the high-flux X-Ray appears to be 9·10⁶ holes/(cm²) (seeEq. (16) below).

In high-flux photon counting used for diagnostic CT, the radiationintensity is I=10¹⁰ photons/(cm²·sec). This radiation produces holes inthe HH valence-band by exciting electrons from this band to theconductance-band. The excited holes have a lifetime of τ_(h)=0.05 μs. Incase that the X-ray radiation is totally absorbed in the detector, thegeneration rate of holes χ is equal to the radiation intensity I timesthe average amount of electron-hole pairs that each photon produces. Theenergy pair-production for CZT is E_(P)=4.4 eV. The average energy E_(A)of the photons emitted from the X-ray tube is about 80 KeV. Accordingly,the average amount of holes produced by each photon times the intensityI of the excitation ionizing radiation (Photons/(cm²·sec) is thegeneration rate and is given by:

$\begin{matrix}\begin{matrix}{\chi = {{I \cdot \frac{E_{A}}{E_{p}}} \approx {10^{10} \cdot \frac{80 \cdot 10^{3}}{4.4}} \approx {10^{10} \cdot 1.8 \cdot 10^{4}}}} \\{= {{1.8 \cdot 10^{14}}\mspace{14mu}{holes}\text{/}\left( {{cm}^{2} \cdot \sec} \right)}}\end{matrix} & {{Eq}.\mspace{14mu}(9)}\end{matrix}$

In the case when none of the holes is collected by the cathode, thesteady-state density of holes N_(h) in the detector produced, by theX-Ray radiation, can be derived from the rate equation:

$\begin{matrix}{{{\chi - \frac{N_{h}}{\tau_{h}}} = \frac{d\left( N_{h} \right)}{dt}}{Or}} & {{Eq}.\mspace{14mu}(10)} \\{{\frac{\left( {{\chi \cdot \tau_{h}} - N_{h}} \right)}{\tau_{h}} = {- \frac{d\left( {{\chi \cdot \tau_{h}} - N_{h}} \right)}{dt}}}{Or}} & {{Eq}.\mspace{14mu}(11)} \\{\frac{dt}{\tau_{h}} = {- \frac{d\left( {{\chi \cdot \tau_{h}} - N_{h}} \right)}{\left( {{\chi \cdot \tau_{h}} - N_{h}} \right)}}} & {{Eq}.\mspace{14mu}(12)}\end{matrix}$

By integrating both sides of Eq. (12):

$\begin{matrix}{\left( {{{??} \cdot \tau_{h}} - N_{h}} \right) = {A \cdot e^{- \frac{\tau}{\tau_{h}}}}} & {{Eq}.\mspace{14mu}(13)}\end{matrix}$

Where A is an integration constant. From the initial conditions at t=0,N_(h)=0.χ·τ_(h) =A  (14)

Substituting the value of A from Eq. (14) into Eq. (13) yields:

$\begin{matrix}{N_{h} = {{??} \cdot \tau_{h} \cdot \left( {1 - e^{- \frac{\tau}{\tau_{h}}}} \right)}} & {{Eq}.\mspace{14mu}(15)}\end{matrix}$

In a steady-state t>>τ_(h) is satisfied and then:N _(h)=χ·τ_(h)=1.8·10¹⁴·5·10⁻⁸=9·10⁶ holes/(cm²)

This means that the X-ray radiation produces 9·10⁶ holes/(cm²) in theHH-band. The transition of electrons from the LH-band to theconduction-band has very low probability. Also, the relaxation ofelectrons from the HH-band to the LH-band is very fast (PS). Thus it maybe assumed that all the holes produced by the X-ray are located in theHH-band. The optical beam intensity to convert most of the HH, producedby the X-Ray radiation, into LH or SOH may be derived, by thecalculations discussed below, to be equal to 0.07 Watt/(cm²). Suchintensity may readily be provided, for example, by a CO₂ laser that iscapable of providing a high power Continuous-Wave (CW) or pulsed beam atwavelengths around 9.4 and 10.6 μm, which are suitable to excite holesfrom the HH-band to the LH-band and to convert the relatively heavyeffective mass of the HH into much lighter effective mass of the LH.

In various embodiments, the conversion of HH into LH or SOH may beachieved by optical direct transition of electrons from the LH-band tothe HH-band. In order to avoid the creation of space-charge in thesemiconductor detector bulk during the irradiation of the detector byhigh flux ionizing radiation, most of the HH produced by the ionizingradiation may be converted into LH by the IR optical excitation.Following is a discussion of the calculation of the optical power neededto convert most of the HH produced by the X-Ray radiation into LH usingIR optical excitation to produce transitions of electrons from theLH-band into the HH-band (transition of holes from the HH band into theLH band) according to an embodiment.

As discussed above, the hole-density in the HH-band is 9·10⁶holes/(cm²). The lifetime of electrons in the HH-band prior to theirrelaxation back into the LH-band is τ_(e-HH)=10⁻¹² sec. To convert mostof the HH into LH, the number of electrons N_(e) excited from theLH-band into the HH-band should be similar to the number of holes N_(h)in the HH-band. Accordingly, from Eq. (16) it follows that:N _(e) =N _(h)=9·10⁶ electrons/(cm²)  (17)

Analogous to Eq. (16) above, for N_(e):N _(e)=χ_(e)·τ_(e-HH)=9·10⁶ electrons/(cm²)

Where χ_(e) is the optical excitation rate of electrons from the LH-bandinto the HH-band. The excitation rate χ_(e) is derived from Eq. (16) andis equal to:

$\begin{matrix}{{??}_{e} = {\frac{N_{e}}{\tau_{e - {HH}}} = {\frac{9 \cdot 10^{6}}{10^{- 12}} = {{9 \cdot 10^{18}}\mspace{14mu}{electrons}\text{/}\left( {{cm}^{2} \cdot \sec} \right)}}}} & {{Eq}.\mspace{14mu}(19)}\end{matrix}$

An optical beam of a CO₂ laser having a wavelength of 10.6 μm can exciteelectrons from the band-edge of the LH to the HH-band without excitingelectrons from the HH-band to the conduction-band. Unlike the ionizingX-Ray radiation in which each photon produces many charge-carriers, eachphoton of the CO₂ laser excites only one electron. When assumingefficiency of 100% for the optical beam, then the number of photons persecond per cm² N_(ph) is equal to the electrons excitation rate χ_(e).Accordingly:N _(ph)=χ_(e)=9·10¹⁸ photons/(cm²·sec)  (20)

The energy of each photon is:

$\begin{matrix}{{E_{p\; h}({eV})} = {\frac{1.25}{\lambda({µm})} = {\frac{1.25}{10.6} = {0.118\mspace{14mu}{eV}}}}} & {{Eq}.\mspace{14mu}(21)}\end{matrix}$

The total intensity of the optical beam is given by:

$\begin{matrix}{{N_{p\; h} \cdot {E_{p\; h}({eV})}} = {{{9 \cdot 10^{18}}\mspace{14mu}{photonss}\text{/}{\left( {{cm}^{2} \cdot \sec} \right) \cdot 0.118}\mspace{14mu}{eV}} = {{{1.06 \cdot 10^{18}}\mspace{14mu}{eV}\text{/}\left( {{cm}^{2} \cdot \sec} \right)} = {\frac{1.06 \cdot 10^{18}}{1.6 \cdot 10^{19}} = {0.066\mspace{14mu}{Watt}\text{/}\left( {cm}^{2} \right)}}}}} & {{Eq}.\mspace{14mu}(22)}\end{matrix}$

The calculations above were made under ideal conditions, accordingly,the calculated power of 0.066 Watt/(cm²) represents the minimum opticalintensity needed for the optical excitation. For example, in an exampleCT system, the length of the detectors arc may be about 80 cm. For 256slice CT, the width of the detector arc that has 1 mm pixels size is256×1 mm=2.56 cm. Accordingly the total detectors area S of thedetectors is 80×2.56 cm²=204.8 cm². The total optical power OP needed toconvert most of the HH into LH over all the detector area S isaccordingly given by:

$\begin{matrix}{{{OP}({Watt})} = {0.066\mspace{14mu}{Watt}\text{/}{\left( {cm}^{2} \right) \cdot {S\left( {\left( {cm}^{2} \right) = {{0.066 \cdot 204.8} = {13.51\mspace{14mu}{Watt}}}} \right.}}}} & {{Eq}.\mspace{14mu}(23)}\end{matrix}$

Even though such optical power may be considered relatively high, itstill may be easily provided by a commercially available CO₂ laser,which is a common and relatively low cost laser. Alternatively, theoptical power may be provided by an array of such lasers.

It may be noted that the excitation optical power may advantageouslysatisfy several conditions. First, a spectral optical range may includewavelengths selected from a wavelength range ensuring efficientconversion of the holes' effective-mass into lighter effective mass.Such a wavelength range may be determined, as mentioned above, by theamount of doping of the semiconductor, the semiconductor composition,the temperature of the semiconductor that determines the position ofquasi Fermi-Level E_(f) (32), and/or the intensity of the flux of theionizing radiation. For CZT radiation detectors, the wavelength range,for example, may start with wavelengths of λ=3.3-5 μm and ends withwavelengths of λ=14-16.5 μm. Second, an optical intensity may beemployed that ensures that the amount of holes, in the split valencebands for which the effective-mass was converted into lightereffective-mass, is similar to the amount of holes in the split valencebands that were generated by the ionizing radiation impinging of theradiation detector. For CZT radiation detectors, the calculations havebeen done under optimal conditions, thus in practice, the IR opticalexcitation-intensity may be equal or higher than the calculated value of0.066 Watt/(cm²). It may be noted that this intensity is substantiallyhigher than needed for producing uniform space charge by ionizing deeplevel hole traps. The relatively high intensity of the optical IRradiation used for the optical excitation according to variousembodiments of the present disclosure is mainly due to the extremelyshort lifetime of holes (in the Pico second range) in the LH and SObands.

FIG. 3 provides a flowchart of a method 300 in accordance with variousembodiments. The method 300, for example, may employ or be performed bystructures or aspects of various embodiments (e.g., systems and/ormethods and/or process flows) discussed herein. In various embodiments,certain steps may be omitted or added, certain steps may be combined,certain steps may be performed concurrently, certain steps may be splitinto multiple steps, certain steps may be performed in a differentorder, or certain steps or series of steps may be re-performed in aniterative fashion. In various embodiments, portions, aspects, and/orvariations of the method 300 may be able to be used as one or morealgorithms to direct hardware (e.g., one or more aspects of theprocessing unit 120) to perform one or more operations described herein.

At 302, wavelengths are determined for converting a group of holes in afirst valance band into at least one other group of holes in a differentvalance band having lower effective mass. The wavelengths may bedetermined as discussed herein, for example based on one or more of thedoping type, doping amount of the semiconductor (the location of thequasi-Fermi level E_(f) inside the split valence bands), the compositionof the semiconductor, the intensity of the ionizing radiation impingingon the radiation detector, and/or the detector temperature, for example.In the illustrated embodiment, at 304, a wavelength or range ofwavelengths is determined for converting heavy holes (HH) to spin-orbitholes (SOH). At 306, a wavelength or range of wavelengths is determinedfor converting light holes (LH) to spin-orbit holes (SOH). At 308, awavelength or range of wavelengths is determined for converting HH toLH.

At 310, detection events are acquired with a radiation detector. Forexample, the radiation detector may include a semiconductor plate. Thedetector may be configured to produce electrical signals in response toabsorption of ionizing radiation in the semiconductor plate, withelectrons and holes are generated responsive to absorption of theionizing radiation. The holes including groups of holes having differenteffective masses for corresponding different valence energy bands (e.g.,HH, LH, and SOH). The detection events, for example, may correspond toone or more of photons that have passed through an object to be imagedas part of a CT scan or an emission from an object to be image as partof an NM scan.

At 312, infrared (IR) radiation is coupled into the semiconductor plateof the radiation detector. The IR radiation may have at least onewavelength (e.g., a wavelength selected at one or more of steps 302-308)selected from a spectral range including wavelengths to which thesemiconductor plate is partially transparent and which are configured toexcite at least some of the holes from a first group at a first valenceenergy band to a second group at a second valence energy band. The holesof the second group have lower effective masses than corresponding holesof the first group. For example, HH may be converted into LH and/or SOH,and/or LH may be converted into SOH in various embodiments. Accordingly,the conversion of holes from the first group to the second group reduceshole mass, increases hole mobility, and thereby reduces polarizationeffects. In some embodiments, the spectral range may include wavelengthsin the range from 3 micrometers (μm) to 16.5 micrometers (μm). The IRradiation may be directed into the semiconductor plates via sidewallsand/or a monolithic cathode electrode that is at least partiallytransparent to the IR excitation radiation, for example, in variousembodiments.

The IR radiation may be optically coupled into the semiconductor plateusing a laser. For example, in the depicted embodiment, at 312, couplingthe IR radiation into the semiconductor plate includes generatingwavelengths of 9.4 micrometers (μm) and 10.6 micrometers (μm) with alaser. Such wavelengths may be substantially different from wavelengthsused to address hole-traps.

Various embodiments discussed herein and illustrated by FIGS. 1-3 may beimplemented in medical imaging systems, such as, for example, SPECT,SPECT-CT, diagnostic CT, spectral CT, PET and PET-CT. In these cases,embodiments discussed herein may be part of the detection units of thesystems mentioned above. It should be understood that while a radiationdetector in accordance with some embodiments may be used either for CTor NM applications, still the very same detector may also be used formultiple applications, such as CT, spectral CT, diagnostic CT, SPECT,PET, SPECT-CT and PET-CT. Various methods and/or systems (and/or aspectsthereof) described herein may be implemented using a medical imagingsystem. For example, FIG. 4 is a schematic illustration of a NM imagingsystem 1000 having a plurality of imaging detector head assembliesmounted on a gantry (which may be mounted, for example, in rows, in aniris shape, or other configurations, such as a configuration in whichthe movable detector carriers 1016 are aligned radially toward thepatient-body 1010). It should be noted that the arrangement of FIG. 4 isprovided by way of example for illustrative purposes, and that otherarrangements (e.g., detector arrangements) may be employed in variousembodiments. In the illustrated example, a plurality of imagingdetectors 1002 are mounted to a gantry 1004. In the illustratedembodiment, the imaging detectors 1002 are configured as two separatedetector arrays 1006 and 1008 coupled to the gantry 1004 above and belowa subject 1010 (e.g., a patient), as viewed in FIG. 4. The detectorarrays 1006 and 1008 may be coupled directly to the gantry 1004, or maybe coupled via support members 1012 to the gantry 1004 to allow movementof the entire arrays 1006 and/or 1008 relative to the gantry 1004 (e.g.,transverse translating movement in the left or right direction as viewedby arrow T in FIG. 4). Additionally, each of the imaging detectors 1002includes a detector unit 1014, at least some of which are mounted to amovable detector carrier 1016 (e.g., a support arm or actuator that maybe driven by a motor to cause movement thereof) that extends from thegantry 1004. In some embodiments, the detector carriers 1016 allowmovement of the detector units 1014 towards and away from the subject1010, such as linearly. Thus, in the illustrated embodiment the detectorarrays 1006 and 1008 are mounted in parallel above and below the subject1010 and allow linear movement of the detector units 1014 in onedirection (indicated by the arrow L), illustrated as perpendicular tothe support member 1012 (that are coupled generally horizontally on thegantry 1004). However, other configurations and orientations arepossible as described herein. It should be noted that the movabledetector carrier 1016 may be any type of support that allows movement ofthe detector units 1014 relative to the support member 1012 and/organtry 1004, which in various embodiments allows the detector units 1014to move linearly towards and away from the support member 1012.

Each of the imaging detectors 1002 in various embodiments is smallerthan a conventional whole body or general purpose imaging detector. Aconventional imaging detector may be large enough to image most or allof a width of a patient's body at one time and may have a diameter or alarger dimension of approximately 50 cm or more. In contrast, each ofthe imaging detectors 1002 may include one or more detector units 1014coupled to a respective detector carrier 1016 and having dimensions of,for example, 4 cm to 20 cm and may be formed of Cadmium Zinc Telluride(CZT) tiles or modules. For example, each of the detector units 1014 maybe 8×8 cm in size and be composed of a plurality of CZT pixelatedmodules (not shown). For example, each module may be 4×4 cm in size andhave 16×16=256 pixels (pixelated anodes). In some embodiments, eachdetector unit 1014 includes a plurality of modules, such as an array of1×7 modules. However, different configurations and array sizes arecontemplated including, for example, detector units 1014 having multiplerows of modules.

It should be understood that the imaging detectors 1002 may be differentsizes and/or shapes with respect to each other, such as square,rectangular, circular or other shape. An actual field of view (FOV) ofeach of the imaging detectors 1002 may be directly proportional to thesize and shape of the respective imaging detector.

The gantry 1004 may be formed with an aperture 1018 (e.g., opening orbore) therethrough as illustrated. A patient table 1020, such as apatient bed, is configured with a support mechanism (not shown) tosupport and carry the subject 1010 in one or more of a plurality ofviewing positions within the aperture 1018 and relative to the imagingdetectors 1002. Alternatively, the gantry 1004 may comprise a pluralityof gantry segments (not shown), each of which may independently move asupport member 1012 or one or more of the imaging detectors 1002.

The gantry 1004 may also be configured in other shapes, such as a “C”,“H” and “L”, for example, and may be rotatable about the subject 1010.For example, the gantry 1004 may be formed as a closed ring or circle,or as an open arc or arch which allows the subject 1010 to be easilyaccessed while imaging and facilitates loading and unloading of thesubject 1010, as well as reducing claustrophobia in some subjects 1010.

Additional imaging detectors (not shown) may be positioned to form rowsof detector arrays or an arc or ring around the subject 1010. Bypositioning multiple imaging detectors 1002 at multiple positions withrespect to the subject 1010, such as along an imaging axis (e.g., headto toe direction of the subject 1010) image data specific for a largerFOV may be acquired more quickly.

Each of the imaging detectors 1002 has a radiation detection face, whichis directed towards the subject 1010 or a region of interest within thesubject.

The collimators 1022 (and detectors) in FIG. 4 are depicted for ease ofillustration as single collimators in each detector head. Optionally,for embodiments employing one or more parallel-hole collimators,multi-bore collimators may be constructed to be registered with pixelsof the detector units 1014, which in one embodiment are CZT detectors.However, other materials may be used. Registered collimation may improvespatial resolution by forcing photons going through one bore to becollected primarily by one pixel. Additionally, registered collimationmay improve sensitivity and energy response of pixelated detectors asdetector area near the edges of a pixel or in-between two adjacentpixels may have reduced sensitivity or decreased energy resolution orother performance degradation. Having collimator septa directly abovethe edges of pixels reduces the chance of a photon impinging at thesedegraded-performance locations, without decreasing the overallprobability of a photon passing through the collimator.

A controller unit 1030 may control the movement and positioning of thepatient table 1020, imaging detectors 1002 (which may be configured asone or more arms), gantry 1004 and/or the collimators 1022 (that movewith the imaging detectors 1002 in various embodiments, being coupledthereto). A range of motion before or during an acquisition, or betweendifferent image acquisitions, is set to maintain the actual FOV of eachof the imaging detectors 1002 directed, for example, towards or “aimedat” a particular area or region of the subject 1010 or along the entiresubject 1010. The motion may be a combined or complex motion in multipledirections simultaneously, concurrently, or sequentially.

The controller unit 1030 may have a gantry motor controller 1032, tablecontroller 1034, detector controller 1036, pivot controller 1038, andcollimator controller 1040. The controllers 1030, 1032, 1034, 1036,1038, 1040 may be automatically commanded by a processing unit 1050,manually controlled by an operator, or a combination thereof. The gantrymotor controller 1032 may move the imaging detectors 1002 with respectto the subject 1010, for example, individually, in segments or subsets,or simultaneously in a fixed relationship to one another. For example,in some embodiments, the gantry controller 1032 may cause the imagingdetectors 1002 and/or support members 1012 to move relative to or rotateabout the subject 1010, which may include motion of less than or up to180 degrees (or more).

The table controller 1034 may move the patient table 1020 to positionthe subject 1010 relative to the imaging detectors 1002. The patienttable 1020 may be moved in up-down directions, in-out directions, andright-left directions, for example. The detector controller 1036 maycontrol movement of each of the imaging detectors 1002 to move togetheras a group or individually. The detector controller 1036 also maycontrol movement of the imaging detectors 1002 in some embodiments tomove closer to and farther from a surface of the subject 1010, such asby controlling translating movement of the detector carriers 1016linearly towards or away from the subject 1010 (e.g., sliding ortelescoping movement). Optionally, the detector controller 1036 maycontrol movement of the detector carriers 1016 to allow movement of thedetector array 1006 or 1008. For example, the detector controller 1036may control lateral movement of the detector carriers 1016 illustratedby the T arrow (and shown as left and right as viewed in FIG. 10). Invarious embodiments, the detector controller 1036 may control thedetector carriers 1016 or the support members 1012 to move in differentlateral directions. Detector controller 1036 may control the swivelingmotion of detectors 1002 together with their collimators 1022. In someembodiments, detectors 1002 and collimators 1022 may swivel or rotatearound an axis.

The pivot controller 1038 may control pivoting or rotating movement ofthe detector units 1014 at ends of the detector carriers 1016 and/orpivoting or rotating movement of the detector carrier 1016. For example,one or more of the detector units 1014 or detector carriers 1016 may berotated about at least one axis to view the subject 1010 from aplurality of angular orientations to acquire, for example, 3D image datain a 3D SPECT or 3D imaging mode of operation. The collimator controller1040 may adjust a position of an adjustable collimator, such as acollimator with adjustable strips (or vanes) or adjustable pinhole(s).

It should be noted that motion of one or more imaging detectors 1002 maybe in directions other than strictly axially or radially, and motions inseveral motion directions may be used in various embodiment. Therefore,the term “motion controller” may be used to indicate a collective namefor all motion controllers. It should be noted that the variouscontrollers may be combined, for example, the detector controller 1036and pivot controller 1038 may be combined to provide the differentmovements described herein.

Prior to acquiring an image of the subject 1010 or a portion of thesubject 1010, the imaging detectors 1002, gantry 1004, patient table1020 and/or collimators 1022 may be adjusted, such as to first orinitial imaging positions, as well as subsequent imaging positions. Theimaging detectors 1002 may each be positioned to image a portion of thesubject 1010. Alternatively, for example in a case of a small sizesubject 1010, one or more of the imaging detectors 1002 may not be usedto acquire data, such as the imaging detectors 1002 at ends of thedetector arrays 1006 and 1008, which as illustrated in FIG. 4 are in aretracted position away from the subject 1010. Positioning may beaccomplished manually by the operator and/or automatically, which mayinclude using, for example, image information such as other imagesacquired before the current acquisition, such as by another imagingmodality such as X-ray Computed Tomography (CT), MRI, X-Ray, PET orultrasound. In some embodiments, the additional information forpositioning, such as the other images, may be acquired by the samesystem, such as in a hybrid system (e.g., a SPECT/CT system).Additionally, the detector units 1014 may be configured to acquirenon-NM data, such as x-ray CT data. In some embodiments, amulti-modality imaging system may be provided, for example, to allowperforming NM or SPECT imaging, as well as x-ray CT imaging, which mayinclude a dual-modality or gantry design as described in more detailherein.

After the imaging detectors 1002, gantry 1004, patient table 1020,and/or collimators 1022 are positioned, one or more images, such asthree-dimensional (3D) SPECT images are acquired using one or more ofthe imaging detectors 1002, which may include using a combined motionthat reduces or minimizes spacing between detector units 1014. The imagedata acquired by each imaging detector 1002 may be combined andreconstructed into a composite image or 3D images in variousembodiments.

In one embodiment, at least one of detector arrays 1006 and/or 1008,gantry 1004, patient table 1020, and/or collimators 1022 are moved afterbeing initially positioned, which includes individual movement of one ormore of the detector units 1014 (e.g., combined lateral and pivotingmovement) together with the swiveling motion of detectors 1002. Forexample, at least one of detector arrays 1006 and/or 1008 may be movedlaterally while pivoted. Thus, in various embodiments, a plurality ofsmall sized detectors, such as the detector units 1014 may be used for3D imaging, such as when moving or sweeping the detector units 1014 incombination with other movements.

In various embodiments, a data acquisition system (DAS) 1060 receiveselectrical signal data produced by the imaging detectors 1002 andconverts this data into digital signals for subsequent processing.However, in various embodiments, digital signals are generated by theimaging detectors 1002. An image reconstruction device 1062 (which maybe a processing device or computer) and a data storage device 1064 maybe provided in addition to the processing unit 1050. It should be notedthat one or more functions related to one or more of data acquisition,motion control, data processing and image reconstruction may beaccomplished through hardware, software and/or by shared processingresources, which may be located within or near the imaging system 1000,or may be located remotely. Additionally, a user input device 1066 maybe provided to receive user inputs (e.g., control commands), as well asa display 1068 for displaying images. DAS 1060 receives the acquiredimages from detectors 1002 together with the corresponding lateral,vertical, rotational and swiveling coordinates of gantry 1004, supportmembers 1012, detector units 1014, detector carriers 1016, and detectors1002 for accurate reconstruction of an image including 3D images andtheir slices.

As discussed herein, various embodiments utilize IR radiation to addressholes (e.g., convert heavy holes to light holes) for improvedperformance of imaging systems. For example, various embodiments addresslow energy tails. A low energy tail as used herein may be understood asa portion of a plot of counts versus energy that includes portions ofthe plot at energies lower than the peak energy. An example of aspectrum 900 including a low energy tail 910 is shown in FIG. 9. FIG. 9is a plot of number of events or counts (on the y-axis or vertical axis)as a function of the energy of the events or counts (on the x-axis orhorizontal axis). As seen in FIG. 9, the spectrum 900 includes a peak920 at which a maximum number of counts are obtained, with the peakoccurring at a peak energy 922. However, for example, due toinconsistencies in detector materials, counts are detected over a rangeof energies lower than the peak energy 922. In the depicted spectrum900, the low energy tail 910 extends over range 914 of lower energiesthan the peak energy 922, and may be attributable, for example, tocharge lost due to the relative immobility of heavy holes at one or moreportions of a detector and corresponding incomplete charge collection.FIG. 9 also schematically illustrates spectrum 902, which representsspectrum 900 after reducing its low energy tail 910 according to variousembodiments discussed herein. Spectrum 902 has a reduced low energy tail912 (relative to low energy tail 910), and has a peak 924 located atenergy peak 928, which is higher than the peak energy of peak 920 ofspectrum 900. The reduced low energy tail 912 of spectrum 902 in energyrange 914 includes fewer counts than tail 910 of spectrum 900 in thesame energy range 914. Spectra 900 and 902 each include the same numberof total counts, which means that counts in the low energy tail ofspectrum 902 in the energy range 914 move out from this range (relativeto spectrum 900) to appear at higher energies. Accordingly, peak 924 ofspectrum 902 is higher than peak 920 of spectrum 900 and contains morecounts. Similarly, spectrum 902, with a reduced low energy tail, hasmore counts within energy window 926 used for the imaging in nuclearimaging than does spectrum 900. Accordingly, the reduced tail ofspectrum 902 makes it more efficient for imaging, and also has theadvantage of reducing the background signal for other isotopes used invarious applications, such as dual isotope imaging.

Various embodiments provide a detector system that includes a blockingcathode (or cathode with a blocking contact), with the detector systemexhibiting improved energy resolution (e.g., by limiting leakagecurrent) as well exhibiting reduced low energy tail (e.g., by opticallyimproving the mobility of holes). In various embodiments, heavy holesare converted into light holes to improve the mobility of the holes.Also, in some embodiments, a radiation detector is provided for whichheavy holes are converted into light holes using optical excitation andtransitions of holes between valance bands of the semiconductor fromwhich the radiation detector is constructed. In various embodiments, theoptical excitations used to convert holes have wavelengths in thespectral range of IR and/or FIR.

In some embodiments, dual isotopes may be used for medical imaging. Itmay be noted that the use of dual isotopes may save time and/or increasethroughput of a medical imaging system. For example, with dual isotopes,imaging may be performed at one time for the two isotopes. In contrast,in standard imaging, imaging for two isotopes would be performed twice(once for each isotope). Further, a relatively long waiting period maybe required between imaging of different isotopes (e.g., to allow theradiation level of the first isotope from the first imaging to decaysignificantly before the second imaging. Accordingly, the use of dualisotope imaging may reduce acquisition time substantially.

It may be noted that radiation detectors used in connection with dualisotope imaging should have relatively high energy resolution as well asrelatively low energy tails for a number of reasons. For example, theemitted energies of the X-ray or Gamma photons of the dual isotopes maybe similar, with high energy resolution beneficial for resolving thedifferent energy peaks of the isotopes. Also, the energy peak of thelower energy (e.g., lower energy corresponding to peak) isotope mayappear in the energy range where the higher energy (e.g., higher energycorresponding to peak) isotope has its low energy tail. Accordingly, thelow energy tail of the higher energy isotope may act as undesirablebackground for the energy peak of the lower energy isotope.

To improve energy resolution, the amount of noise may be reduced. It maybe noted that two of the main noise sources in a detector are theleakage current through the detector and the capacitance of thedetector. Both the leakage current through the detector and the detectorcapacitance are proportional to the area of the pixels. Accordingly,pixel size may be reduced to reduce to detector noise and to improveenergy resolution. However, reduction of pixel size may have certaindrawbacks. For example, implementation of reduced pixel size may bedifficult due to the requirement of additional pixels and electronicchannels. Also, reduction of pixel size increases the number ofcharge-sharing events and makes the low energy tail larger. Accordingly,increasing the energy resolution by decreasing pixel size causes anincrease in the low energy tail which degrades detector performance(e.g., for use in dual isotope imaging due to the increase in the lowenergy tail). Further, to reduce the low energy tail, a method of chargesharing recovery including summing the charge sharing signals receivedfrom adjacent pixels may be applied for charge sharing events. However,summing the charge sharing events received from adjacent pixelsincreases the noise and degrades the energy resolution.

It may be noted that a significant cause of the formation of low energytails in the spectrum of a detector is the dependency of the signalsreceived from the pixels on the absorption depth within the bulk of thedetector, or on the depth of interaction (DOI). One potential method ofreducing low energy tails by eliminating or reducing the dependency onDOI is to employ small pixels to produce a “small pixel effect.” Toimprove the attraction of electrons to the small pixelated anodes, asteering non-collecting grid is added between the pixels, with the gridbiased by an electrical potential that is lower than the pixels'electrical potential. However, the combination of small pixels with asteering grid has the same drawbacks discussed herein in connection withreduction of pixel size. Alternatively, the electrical field of aradiation detector may be internally shaped to produce asmall-pixel-like effect. Such an approach, however, does not improveenergy resolution.

To improve energy resolution, a blocking cathode contact may beprovided. The blocking cathode may be used to reduce leakage current.However, the reduction of leakage current and improvement in energyresolution may come at the expense of increased low energy tails,degrading performance. For example, blocking contacts create a depletionregion near the cathode. The depletion region has a relatively highelectrical resistance that is higher than the electrical resistance ofthe detector bulk. Accordingly, the resistance of the depletion regionand the resistance of the detector bulk are electrically connected inseries and from a voltage divider. For such a voltage divider, arelatively large (e.g., greater than ½) fraction of the bias applied onthe detector is dropped on the depletion region and a relatively smallerfraction is dropped on the remainder of the detector bulk. Thisdistribution produces a high electrical field in the depletion regionnear the cathode and the rest of the detector volume has a reducedelectrical field (e.g., relative to the field in the depletion region).The reduced electrical field in the detector volume results inrelatively larger low energy tails, for example, due to lengthened drifttime of the charge carriers toward the contacts, which is longer thanthe charge-carriers' lifetime. This results in incomplete chargecollection due to relatively high recombination in the detector volumethat has the reduced field, resulting in lower energies for acquiredevents, thereby producing a relatively large low-energy tail.

Accordingly, to address certain drawbacks of conventional imagingsystems, various embodiments provide radiation detectors having blockingcathode contacts that exhibit both improved energy resolution (e.g., bylimiting leakage current), as well as reduced low energy tails (e.g., byoptically improving mobility of holes). Additionally or alternatively,various embodiments control or otherwise limit the level of opticalirradiation provided to a detector and/or the locations at which theirradiation is provided for improved imaging (e.g., by reducing theeffect of low energy tails).

FIGS. 5-7 schematically illustrate the energy band structures E_(n)(FIG. 5), the electrical field E (FIG. 6), and the number of absorbedphotons N (FIG. 7) for a semiconductor detector (e.g., semiconductorplate 110 or semiconductor plate 610), as a function of the coordinateZ. In FIGS. 5-7, Z represents a distance in the thickness or width of adetector extending between a cathode 112 and anodes 114. With thethickness or width of the detector represented by D, a Z coordinate of 0is located at the cathode, and a Z coordinate of D is located at theanodes.

FIG. 5 schematically illustrates an energy band structure 2000 as afunction of coordinate Z in a range between the cathode 112 (Z=0) andthe anodes 114 (Z=D). The energy band structure 2000 includes aconduction band E_(C) 2008, a valance band Ev 2010, and a Fermi-levelband E_(F) 2012. FIG. 5 also depicts a potential barrier 2002, apotential 2014 (q(ϕ_(i)−V)) for semiconductor plate 116 (e.g.,semiconductor plate 116 from FIG. 1), and a reverse potential qV 2016for semiconductor plate 116 that includes the barrier 2002 (qϕ_(b))formed at an interface 2006 between the semiconductor plate 116 andcathode 112, which limits the leakage current and thus improves theenergy-resolution of semiconductor plate 110. Energy band structure 2000is illustrated in FIG. 5 under a reverse bias, with a negativeelectrical bias applied to the cathode 112 and a positive electricalbias applied to the semiconductor plate 116. The potential betweencathode 112 and semiconductor plate 116 in FIG. 5 may be quantified as Vand, accordingly, Fermi-level 2004 in equilibrium is shifted as a resultof the reverse bias by an amount of qV 2016 at the metal/semiconductor(MS) interface 2006 between the cathode 112 and the semiconductor plate116, with the shifted equilibrium depicted as Fermi-level E_(F) 2012under reverse bias. It may be noted that potentials 2002 and 2014 aremeasured from levels 2022 and 2012 to vacuum level 2024, respectively.

Under reverse bias, as seen in FIG. 5, the conductance band 2008 and thevalance band 2010 are curved in a shape similar to the shape of theFermi-level E_(F) 2012, with the level increasing as the curves approachthe cathode (or approaching Z=0). Accordingly, a depletion region 2018is formed between the metal of the cathode 112 (or Z=0) and boundary2020. The remainder of the detector bulk, or portion of the detectorbulk in the region between the edge of depletion region 2018 at boundary2020 and anode 114 (Z−=D), may be understood as a non-depleted region2026. Depletion region 2018 and non-depletion region 2026 may beunderstood as having electrical resistances that are connected inseries, with the electrical resistance of depletion region 2018considerably higher than the electrical resistance of the non-depletedregion 2026.

As discussed herein regarding the division of voltage, the disparitybetween resistances of the depletion region 2018 and the non-depletedregion 2026 causes a large fraction of the voltage (bias) applied on thedetector between the cathode 112 and the anodes 114 to drop on depletionregion 2018, and a relatively smaller fraction of the voltage to bedropped on the rest of the detector volume in the non-depleted region2026. Accordingly, the potential on the depletion region 2018 is higherthan that for the non-depleted region 2026, and also has a strongervoltage-gradient.

FIG. 6 provides a graph 2050 that shows, as a function of coordinate Zin the range between cathode 112 (Z=0) and anodes 114 (Z=D), anelectrical field E 2052. The electrical field 2052 shown in graph 2050is proportional to the gradient of the potential (energy E_(n)) ofconductance band 2008 in FIG. 5. Accordingly, the electrical field 2052in graph 2050 has a relatively higher value for depletion region 2018,which drops down to a relatively lower value for the non-depleted region2026. For the non-depleted region 2026, the electrical field 2052 isclose to constant.

As seen in graph 2050 of FIG. 6, the use of a blocking contact for thecathode 112 produces a relatively high electrical field near the cathode112 in the depletion region 2018, with the remainder of the detectorvolume in the non-depleted region 2026 having a reduced electrical field(e.g., reduced relative to the field near the cathode 112. The reducedlevel of the electrical field 2052 in the non-depleted region 2026 maycause a relatively large low energy tail due to incomplete chargecollection caused by the high recombination in the non-depleted region2026 of semiconductor plate 110 in which the electrical field isreduced.

The absorption of a photon in the detector creates two charge carrierclouds. One of the clouds is an electron cloud and the other cloud is aholes cloud. Each of these clouds has an electrical charge of magnitudeQ₀. The electron cloud, having a negative electrical charge, isattracted by the positive voltage of anodes 114 and is drifted from thelocation where it was created toward the anodes 114. Similarly, theholes cloud, having positive electrical charge, is attracted by thenegative voltage of cathode 112 and is drifted from the location whereit was created toward cathode 112.

During the times t_(e) and t_(h) that the electrical charges of theelectron clouds and the holes clouds are drifted toward the anodes 114and the cathode 112, respectively, the clouds lose electrical charge dueto the recombination process. The amounts of electron charge Q_(e) andhole charge Q_(h) arriving to the cathode 112 and the anodes 114 aregiven by Equation (24) and (25), respectively:Q _(e) =Q ₀ ·e ^(−t) ^(e) ^(/τ) ^(e)   (24)Q _(h) =Q ₀ ·e ^(−t) ^(h) ^(/τ) ^(h)   (25)

Q_(e) is the electrical charge of electrons arriving to the anodes 114,Q_(h) is the electrical charge of holes arriving to cathode 112, and Q₀is the electrical charge of electrons or holes produced by absorption ofa photon in the semiconductor plate 110. t_(e) and t_(h) are the drifttimes of electrons and holes from the absorption location of the photonto anodes 114 and cathode 112, respectively, and τ_(e) and τ_(h) are thelifetimes of electrons and holes, respectively.

The longer the drift times t_(e) and t_(h), the smaller are theelectrical charges Q_(e) and Q_(h) arriving to anodes 114 and cathode112, respectively. It may be noted that the drift time depends on thevelocity V and the traveling distance Z of the electrical charges fromthe photon absorption location to their corresponding electricalcontacts (e.g., anode 114 or cathode 112). The drift times t_(e) andt_(h) are given by Equations (26) and (27), respectively:

$\begin{matrix}{t_{e} = {\frac{Z_{e}}{V_{e}} = \frac{Z_{e}}{\mu_{e} \cdot E}}} & {{Eq}.\mspace{14mu}(26)} \\{t_{h} = {\frac{Z_{h}}{V_{h}} = \frac{Z_{h}}{\mu_{h} \cdot E}}} & {{Eq}.\mspace{14mu}(27)}\end{matrix}$

where Z_(e) and Z_(h) are the drift distances of electrons and holesfrom the absorption location of the photons to the anodes 114 and thecathode 112, respectively, V_(e) and V_(h) are the drift velocity of theelectrons and holes, respectively, μ_(e) and μ_(h) are the mobilities ofthe electrons and holes, respectively, and E is the value of theelectrical field.

Substituting the mathematical expressions for the drift times t_(e) andt_(h) from Equations (26) and (27) into Equations (24) and (25) yieldsEquations (28) and (29):Q _(e) =Q ₀ ·e ^(−z) ^(e) ^(/(μ) ^(e) ^(·τ) ^(e·E))   (28)Q _(h) =Q ₀ ·e ^(−z) ^(h) ^(/(μ) ^(h) ^(·τ) ^(h·E))   (29)

where Z_(e) and Z_(h) are the drift distance of electrons and holes fromthe absorption location of the photon to the anodes 114 and the cathode112, respectively.

From Equations (28) and (29), it may be seen that the lower theelectrical field becomes, the lower becomes the electrical charge ofelectrons and holes arriving to the contacts (e.g., anodes 114 andcathode 112) of the semiconductor plate 110. Accordingly, when photonsare absorbed in the non-depleted region 2026 (which has a reducedelectrical field), the charge produced by such photons in thenon-depleted region 2026 will suffer from charge loss prior to arrivalat the contacts due to charge loss.

It may be noted that the electrical-charge Q induced on the anode 114 bythe moving charges of the electron and holes clouds, in the absence of asmall-pixel effect, may be given by the Hecht equation as follows:

$\begin{matrix}{Q = {Q_{0} \cdot \left\{ {{\frac{\left( {\mu_{e} \cdot \tau_{e} \cdot E} \right)}{D}\left\lbrack {1 - e^{{- Z}/{({\mu_{e} \cdot \tau_{e} \cdot E})}}} \right\rbrack} + {\frac{\left( {\mu_{h} \cdot \tau_{h} \cdot E} \right)}{D} \cdot \left\lbrack {1 - e^{{- {({D - Z})}}/{({\mu_{h} \cdot \tau_{h} \cdot E})}}} \right\rbrack}} \right\}}} & {{Eq}.\mspace{14mu}(30)}\end{matrix}$where D is the thickness of the semiconductor plate 110, and Z is thecoordinate (depth) at which the photon is absorbed. For a highelectrical field E the following mathematical relationships exist:

$\frac{Z}{\left( {\mu_{e} \cdot \tau_{e} \cdot E} \right)} ⪡ {1\mspace{14mu}{and}\mspace{14mu}\frac{Z}{\left( {\mu_{h} \cdot \tau_{h} \cdot E} \right)}} ⪡ 1.$Under those conditions, the induced charge Q on the anode 114 inEquation (30) is about Q₀. That particular case may be referred to as acomplete charge collection case.

It may be noted that the electrical field E 2052 in the non-depletedregion 2026 is reduced, resulting in the following conditions:

$\frac{Z}{\left( {\mu_{e} \cdot \tau_{e} \cdot E} \right)} > {1\mspace{14mu}{and}\mspace{14mu}\frac{Z}{\left( {\mu_{h} \cdot \tau_{h} \cdot E} \right)}} > 1.$Under these conditions, the induced charge Q on the anode 114 inEquation (30) is smaller than Q₀. Such a case may be referred to asincomplete charge collection. In addition to the fact that thecollection is incomplete, an additional drawback is that the collectedcharge Q in anode 114 from the non-depleted region 2026 depends on theabsorption depth Z of the photon in semiconductor plate 110.

It may be noted that the dependency of the induced charge Q on theabsorption depth Z in the non-depleted region 2026 becomes stronger whenthe semiconductor plate 110 is made of a semiconductor such as CdZnTe(CZT) for which the mobility of the holes is small and thus

$\frac{Z}{\left( {\mu_{h} \cdot \tau_{h} \cdot E} \right)} ⪢ 1.$In this case, the holes practically do not contribute to the inducedcharge Q on the anode 114, and Q depends only on the induced charge ofthe electrons.

It may also be noted that the dependency of the induced charge Q on theabsorption depth Z in the non-depleted region 2026 means that thesemiconductor plate 110 measures different induced charges Q for similarphotons that are absorbed at different depths Z. The induced charge Q isproportional to the energy of the photon measured by the semiconductorplate 110. Accordingly, the energies of the photons measured by thesemiconductor plate 110 also depend on the corresponding photons'absorption depths Z in the non-depleted region 2026. In this situation,for similar photons having the same energy that are absorbed indifferent depths Z in the non-depleted region 2026, different energiesare measured by the semiconductor plate 110, adversely affectingdetector performance. It may further be noted that the dependency of theenergy measured by the semiconductor plate 110 on the absorption depth Zis the main reason for the formation of the low energy tail in thespectrum of the semiconductor plate 110, since, even when all theabsorbed photons are having the same energy, a different energy ismeasured by the semiconductor plate 110 for each different absorptiondepth.

Accordingly, the reduced electrical field in the non-depleted region2026 results in the non-depleted region being a significant cause forthe formation of the low energy tail in the spectrum of thesemiconductor plate 110. Accordingly, the following discussion willprimarily focus on the non-depleted region 2026 and how to addressissues of the non-depleted region 2026. Addressing the issues of thenon-depleted region 2026 is an effective way of improving detectorperformance for at least 3 reasons: (1) the non-depleted region 2026 isthe problematic region and is the main region that produces low energytails in detector spectrums; (2) in the other region of thesemiconductor plate 110 (or depletion region 2018), the electrical fieldis relatively strong and, accordingly, any electron charge produced byan event (e.g., absorption of a photon) created in the depletion region2018 is transferred, generally instantly, by the strong field, to thenon-depleted region 2026; and (3) as shown in FIG. 7 and discussedbelow, most of the events are created in the non-depleted region 2026.

FIG. 7 illustrates a graph 2100 including an absorption curve 2102representing the number of photons absorbed in the semiconductor plate110 as a function of the distance Z from cathode 112. The depictedcathode 112 receives N₀ photons. The depicted absorption curve 2102 isdescribed by the function given by Equation (31):N=N ₀·[1−e ^(−α·Z)]  (31)where α is the absorption coefficient for a given photon energy, N isthe number of photons absorbed at a distance from the cathode 12specified as Z, and N₀ is the number of photons impinging the cathode112 or being received by the cathode 112. Accordingly, it may be seenfrom Equation 31, as also mentioned above, that the number of photons Nabsorbed in the non-depleted region 2026 is much larger than the numberof photons N absorbed in the depletion region 2018.

FIG. 8 illustrates a graph 2200 including a curve 2202 of the calculatednormalized induced charge (or Q(z)/Q₀) on the anode 114 of semiconductorplate 110 made of CZT, as a function of the Depth-Of-Interaction (DOI) Zin the non-depletion region 2026. It may be noted that the distance Z inFIG. 8 is distance from an anode, in contrast to the distance Z in FIGS.5-7 which was distance from the cathode. It may be noted that thecalculations mentioned above, for the semiconductor plate 110, are made,for an example embodiment discussed herein, under the followingconditions and parameters: detector thickness D=0.5 cm, voltage appliedon the detector V=600V, (μ_(e)·τ_(e))=10⁻³ cm²/(V sec), and(μ_(h)·τ_(h))=10⁻⁵ cm²/(V sec).

Under these example conditions, if the electrical field in semiconductorplate 110 were to be uniform, then the electrical field would beE=V/D=600/0.5=1200V/cm. However, since for the depicted example theelectrical field in the non-depletion region 2026 is reduced by about afactor of 2.5, the electrical field in the non-depletion region 2026 isabout 480V/cm. In this case, a free path lengthL_(e)=(μ_(e)·τ_(e)·E)=0.48 cm is obtained for electrons, and a free pathlength L_(h)=(μ_(e)·τ_(e)·E)=0.0048 cm is obtained for holes.Substituting the derived values above into Equation (30) and dividingthe equation by Q₀ for normalization gives Equation (32):

$\begin{matrix}{\frac{Q(Z)}{Q_{0}} = {\left\{ {{\frac{L_{e}}{D}\left\lbrack {1 - e^{{- Z}/L_{e}}} \right\rbrack} + {\frac{L_{h}}{D} \cdot \left\lbrack {1 - e^{{- {({D - Z})}}/L_{h}}} \right\rbrack}} \right\} = \left\{ {{\frac{0.48}{0.5}\left\lbrack {1 - e^{{- Z}/0.48}} \right\rbrack} + {\frac{0.0048}{0.5} \cdot \left\lbrack {1 - e^{{- {({0.5 - Z})}}/0.0048}} \right\rbrack}} \right\}}} & {{Eq}.\mspace{14mu}(32)}\end{matrix}$

It may be noted that, in some embodiment, the non-depleted region 2026may start at about Z=0.13 cm from the cathode 112 and ends at aboutZ=0.4 cm from the cathode 112, which may be about 0.1 cm prior to theanode 114, where the small pixel effect may start to take place.

It may be noted that the free path length L_(h)=(μ_(e)·τ_(e)·E)=0.0048cm for holes in a CZT detector may be so small that the holespractically do not contribute to the normalized induced charge inEquation (32). In this case, Equation (32) may be reduced to thecontribution of electrons only and have the reduced form as shown byEquation (33):

$\begin{matrix}{\frac{Q(Z)}{Q_{0}} = {\left\{ {\frac{L_{e}}{D}\left\lbrack {1 - e^{{- Z}/L_{e}}} \right\rbrack} \right\} = \left\{ {\frac{0.48}{0.5}\left\lbrack {1 - e^{{- Z}/0.48}} \right\rbrack} \right\}}} & {{Eq}.\mspace{14mu}(33)}\end{matrix}$

It may be noted that curve 2202 in the graph 2200 of FIG. 8 shows thevalue of a normalized induced charge Q(Z)/Q₀ in Equation (33), as afunction of the event depth Z. Coordinate Z in FIG. 8 is the distance ofthe location, where the event was created, from the anode 114, and thenormalized induced charge Q(Z)/Q₀ is considered as induced by theelectrons only when there is no significant contribution to thenormalized induced charge Q(Z)/Q₀ by the holes, as occurs in CZTdetectors.

It may be seen from the depicted curve 2202 that, for the illustratedexample, the normalized induced charge Q(Z)/Q₀ starts with value ofabout 0.22 at Z=0.13 and increases generally monotonically to be about0.54 at Z=0.4. Accordingly, the energy E_(P) of the photons absorbed forthe example case are distributed monotonically in the energy rangestarting with energy of 0.22 E_(P) and ending with energy 0.54 E_(P).This energy distribution as measured by the semiconductor plate 110produces a significant low energy tail in the spectrum of semiconductorplate 110.

Increasing the electrical field in the non-depleted region 2026 byincreasing the electrical bias across the semiconductor plate 110 may beattempted. However, such an attempted solution is impractical becauseincreasing the field in the non-depleted region 2026 to a desired levelrequires increasing of the bias voltage across the semiconductor plate110 to a level that is so high that it may cause electrical breakdownand/or electrical sparks that may damage the semiconductor plate 110.Accordingly, various embodiments address the issue of the low energytail produced by the relatively small size of the field in thenon-depleted region 2026 by improving the mobility μ_(h) of the holes tobe more similar to the mobility μ_(e) of the electrons. Variousembodiments increase the mobility of the holes by conversion ofheavy-holes (H-H) into light-holes (L-H). It may be noted that variousaspects relating to converting H-H into L-H is also discussed herein inconnection with FIGS. 1-4.

The conversion of H-H into L-H in various embodiments is performed byirradiating the non-depleted region 2026 of semiconductor plate 110 withIR or FIR radiation, using for example, techniques similar to thosediscussed in connection FIGS. 1 and 3. For CZT, the mobility μ_(h) ofthe converted L-H may be similar to the mobility μ_(e) of the electrons.In such a case, Equation (32) for the normalized induced charge

$\frac{Q(Z)}{Q_{0}}$as a function of the distance Z of the event creation-point (DOI) fromthe anode 114 may be represented by the following:

$\begin{matrix}{\frac{Q(Z)}{Q_{0}} = {\left\{ {{\frac{L_{e}}{D}\left\lbrack {1 - e^{{- Z}/L_{e}}} \right\rbrack} + {\frac{L_{h}}{D} \cdot \left\lbrack {1 - e^{{- {({D - Z})}}/L_{h}}} \right\rbrack}} \right\} = {\frac{0.48}{0.5}\left\{ {\left\lbrack {1 - e^{{- Z}/0.48}} \right\rbrack + \left\lbrack {1 - e^{{- {({0.5 - Z})}}/0.48}} \right\rbrack} \right\}}}} & {{Eq}.\mspace{14mu}(34)}\end{matrix}$where the mathematical expression

$\frac{0.48}{0.5}\left\lbrack {1 - e^{{- Z}/0.48}} \right\rbrack$is the calculated contribution of electrons to the normalized charge

$\frac{Q(Z)}{Q_{0}}$as a function of the distance Z, which is shown by the curve 2202 ofFIG. 8, and the mathematical expression

$\frac{0.48}{0.5} \cdot \left\lbrack {1 - e^{{- {({0.5 - Z})}}/0.48}} \right\rbrack$is the calculated contribution of holes to the normalized charge

$\frac{Q(Z)}{Q_{0}}$as a function of the distance Z, which is shown by the curve 2204 ofFIG. 8.

With continued reference to Equation (34), the total contribution of theelectrons and the holes to the normalized charge

$\frac{Q(Z)}{Q_{0}}$as a function of the distance Z is shown by curve 2206 in FIG. 8. Asseen in FIG. 8, the normalized charge

$\frac{Q(Z)}{Q_{0}}$has a minimum value of about 0.7 at Z=0.4 centimeters, and has a maximumvalue of about 0.77 at Z=0.25 centimeters. In between those two points,the normalized charge

$\frac{Q(Z)}{Q_{0}}$varies monotonically. The induced normalized charge

$\frac{Q(Z)}{Q_{0}}$is proportional to the energy of the photon as measured by thesemiconductor plate 110. As a result, the energy E_(P) of the photonsabsorbed in the non-depleted region 2026 is measured by thesemiconductor plate 110 as energies that are distributed monotonicallyover a range of energy between 0.7 E_(P) and 0.77 E_(P).

As discussed herein, for the semiconductor plate 110 without H-Hconversion into L-H, the measured energies of the events in thenon-depleted region 2026 (as shown by curve 2202 of FIG. 8) are withinan energy range of E_(P)*(0.54−0.22) or 0.32 E_(P), while, for thesemiconductor plate 110 with H-H conversion into L-H, according tovarious embodiments, the measured energies of the events in thenon-depleted region 2026 (as shown by curve 2206 of FIG. 8) are withinan energy range of E_(P)*(0.77−0.7) or 0.07 E_(P). As such, the energydistribution as measured by the semiconductor plate 110 with H-Hconversion into L-H is narrower by a factor of 0.32/0.07=4.6 compared toa measured energy distribution without H-H conversion. Accordingly, thelow energy tail produced in connection with H-H into L-H conversionaccording to various embodiments, is about 4.6 times lower than the lowenergy tail produced without H-H into L-H conversion. Reduction of thelow energy tail by a factor of 4.6 may provide a dramatic improvement inperformance.

It may be noted that further reduction of the energy tail in thespectrum of the semiconductor plate 110 using H-H into L-H conversion invarious embodiments may be achieved by adjusting the number of H-H thatare converted into L-H. For example, Equation (23) discussed hereinprovides an example of how to derive an intensity of an opticalexcitation beam for converting all the H-H into L-H. In someembodiments, an intensity less than that needed for converting all theH-H into L-H may be employed, resulting in only part of the H-H beingconverted into L-H. A fraction K of the H-H that are converted into L-Hmay be adjusted by adjusting the intensity of the optical excitationbeam to correspond with or produce any desired value of K between 0 to1.

Curve 2208 of FIG. 8 shows a calculated contribution of holes to thenormalized charge

$\frac{Q(Z)}{Q_{0}}$as a function of the distance Z when only 0.9 of the H-H are convertedinto L-H (or when the fraction factor K=0.9). In such a case, themathematical expression for the contribution of the L-H to thenormalized induced charge

$\frac{Q(Z)}{Q_{0}}$as a function of the distance Z from anode 114 is0.9·[1−e^(−(0.5−Z)/0.48)]. For such a case, the total induced charge

$\frac{Q(Z)}{Q_{0}}$as a function of coordinate Z may be given by:

$\begin{matrix}{\frac{Q(Z)}{Q_{0}} = {\left\{ {{\frac{L_{e}}{D}\left\lbrack {1 - e^{{- Z}/L_{e}}} \right\rbrack} + {K \cdot \frac{L_{h}}{D} \cdot \left\lbrack {1 - e^{{- {({D - Z})}}/L_{h}}} \right\rbrack}} \right\} = {\frac{0.48}{0.5}\left\{ {\left\lbrack {1 - e^{{- Z}/0.48}} \right\rbrack + {0.9 \cdot \left\lbrack {1 - e^{{- {({0.5 - Z})}}/0.48}} \right\rbrack}} \right\}}}} & {{Eq}.\mspace{14mu}(35)}\end{matrix}$

Curve 2210 of FIG. 8 depicts an example of a normalized charge asspecified by Equation 35. Curve 2210 depicts the total contribution ofthe electrons and the holes to the normalized charge

$\frac{Q(Z)}{Q_{0}}$as a function of the distance Z, when the fraction factor K of thenumber of H-H being converted into L-H is equal to 0.9 using Equation35. As shown by the depicted curve 2210 for the illustrated example, thenormalized charge

$\frac{Q(Z)}{Q_{0}}$has a minimum value of about 0.69 at Z=0.13 centimeters and has amaximum value of about 0.74 at Z=0.3 centimeters. In between those twopoints, the normalized charge varies monotonically. The inducednormalized charge

$\frac{Q(Z)}{Q_{0}}$is proportional to the energy of the photon measured by thesemiconductor plate 110. As such, the energy E_(P) of the photonsabsorbed in the non-depleted region 2026 is measured by thesemiconductor plate 110 as energies that are distributed monotonicallyin an energy range between 0.69 E_(P) and 0.74 E_(P).

As discussed herein, for semiconductor plate 110 having H-H into L-Hconversion without adjusted K fraction (e.g., K=1), the measuredenergies of the events in the non-depleted region 2026 (as shown bycurve 2206) are within an energy range of 0.07 E_(P). However, with theadjusted K fraction of K=0.9, the measured energies of the events in thenon-depleted region 2026 (as shown by curve 2210) are within energyrange of E_(P)*(0.74−0.69)=0.05 E_(P). Accordingly, the energydistribution for the example embodiment with K=0.9 is narrower by afactor of 0.07/0.05=1.4 with respect to use of K=1 for the depictedexample. It may further be noted that, for the illustrated example, theenergy distribution using K=0.9 is narrower by a factor of 0.32/0.05=6.4with respect to the example without H-H into L-H conversion.Accordingly, the low energy tail produced by the semiconductor plate 110when using H-H into L-H conversion with adjustable fraction factor K=0.9for the illustrated example is 1.4 times lower than the low energy tailproduced by with H-H into L-H conversion without an adjusted factor(e.g., K=1.0), and is 6.4 times lower than the low energy tail producedwithout H-H into L-H conversion. It may be noted that the particularvalues used in the above discussion are meant by way of example forillustrative purposes. Other values for the adjusted K fraction or Kfactor and/or other relative levels of improvements in narrowing theenergy range may be utilized or achieved in other embodiments.Generally, the K fraction or K factor may be selected, configured, ordesigned such that the measured energies are within a relatively smallrange, and/or that the curve corresponding to the measured energies(e.g., curve 2210) is within a desired level of flatness (e.g., aperfectly horizontal curve would represent no variation in measuredenergies throughout the non-depleted region 2026).

Various embodiments include a blocking cathode contact for reducingleakage current to improve the energy-resolution of semiconductor plate110 (see FIG. 1 and related discussion). In addition, the optical IR orFIR excitation improves the mobility of holes by the conversion of H-Hinto L-H to compensate for electron charge loss and, accordingly,produces reduced low energy tails. Unlike various previously attemptedapproaches, both goals (e.g., reducing leakage current and improvinghole mobility or reducing low energy tails to improve detectorperformance) are achieved simultaneously, with improvedenergy-resolution provided by a blocking contact and reduced low energytails provided by improved H-H into L-H conversion. It may be notedthat, in various embodiments, the illumination of the non-depletedregion 2026 of semiconductor plate 110, by IR or FIR optical radiation,may be performed by coupling the optical radiation via side walls 116 ofsemiconductor plate 110 or by other methods, such as those mentioned anddiscussed in connection with FIGS. 1 and 3 herein. In some embodiments,in order to reduce the optical power of the excitation radiation, onlythe non-depleted region 2026 may be irradiated by the optical beam viasidewalls 116 of the semiconductor plate 110.

FIG. 10 provides a flowchart of a method 500 in accordance with variousembodiments. The method 500, for example, may employ or be performed bystructures or aspects of various embodiments (e.g., systems and/ormethods and/or process flows) discussed herein. In various embodiments,certain steps may be omitted or added, certain steps may be combined,certain steps may be performed concurrently, certain steps may be splitinto multiple steps, certain steps may be performed in a differentorder, or certain steps or series of steps may be re-performed in aniterative fashion. In various embodiments, portions, aspects, and/orvariations of the method 500 may be able to be used as one or morealgorithms to direct hardware (e.g., one or more aspects of theprocessing unit 120 or the processing unit 620) to perform one or moreoperations described herein.

At 502, wavelengths are determined for converting a group of holes in afirst valance band into at least one other group of holes in a differentvalance band having lower effective mass. The wavelengths may bedetermined as discussed herein, for example based on one or more of thedoping type, doping amount of the semiconductor (the location of thequasi-Fermi level E_(f) inside the split valence bands), the compositionof the semiconductor, the intensity of the ionizing radiation impingingon the radiation detector, and/or the detector temperature, for example.The wavelengths may be determined in various embodiments using one ormore of the steps 302, 304, 306, or 308 discussed in connection withFIG. 3.

Returning to FIG. 10, at 504, an intensity for IR radiation to becoupled into the detector is determined. For example, an opticalintensity (Eq. 22) and/or an optical power (Eq. 23) may be determinedfor converting all heavy holes under ideal conditions. In variousembodiments, the intensity used for the IR radiation to be coupled intothe detector may be less than such an intensity determined forconverting all heavy holes. For example, the intensity may be selectedto provide a desired level of flatness for a curve that plots normalizedinduced charge as a function of depth of interaction (see, e.g., FIG. 8and related discussion). By improving the flatness of the curve of FIG.8, the low energy tail may be reduced. For example, the determinedintensity may be 90% of the calculated intensity for converting allheavy holes under ideal conditions (or have a K factor or K fraction of0.9). As another example, the determined intensity may be 80% of thecalculated intensity (or have a K factor or K fraction of 0.8). As onemore example, the determined intensity may be 50% of the calculatedintensity (or have a K factor or K fraction of 0.5). As discussedherein, for some detectors, under ideal conditions, an intensity of0.066 watt/square centimeter may be required to convert all heavy holes.Accordingly, the intensity selected to improve the flatness of the curveof FIG. 8 in various embodiments is less than 0.066 watt/squarecentimeter in various embodiments.

At 506, detection events are acquired with a radiation detector. Forexample, the radiation detector may include a semiconductor plate. Thedetector may be configured to produce electrical signals in response toabsorption of ionizing radiation in the semiconductor plate, withelectrons and holes are generated responsive to absorption of theionizing radiation. The holes including groups of holes having differenteffective masses for corresponding different valence energy bands (e.g.,HH, LH, and SOH). The detection events, for example, may correspond toone or more of photons that have passed through an object to be imagedas part of a CT scan or an emission from an object to be image as partof an NM scan.

At 508, infrared (IR) radiation is coupled into the semiconductor plateof the radiation detector. The IR radiation is coupled into thesemiconductor plate during acquisition of the detection events toprovide at least partial conversion of heavy holes and to reduce the lowenergy tail. The IR radiation may have at least one wavelength (e.g., awavelength selected at one or more of steps 502, 302-308) selected froma spectral range including wavelengths to which the semiconductor plateis partially transparent and which are configured to excite at leastsome of the holes from a first group at a first valence energy band to asecond group at a second valence energy band. The holes of the secondgroup have lower effective masses than corresponding holes of the firstgroup. For example, HH may be converted into LH and/or SOH, and/or LHmay be converted into SOH in various embodiments. As discussed herein,some embodiments, the spectral range may include wavelengths in therange from 3 micrometers (μm) to 16.5 micrometers (μm). In the depictedembodiments, the IR radiation is be directed into the semiconductorplates via a first portion of at least one sidewall of a detector, butnot coupled into a second portion of the at least one sidewall. Thesidewalls of the detector extend between a cathode electrode and anodeelectrodes. For example, the sidewalls may extend between first andsecond opposed surfaces, with the cathode electrode disposed on thefirst surface, and the anode electrodes disposed on the second surface.It may be noted that in various embodiments, the cathode electrode is ablocking electrode having a non-symmetrical current response. Forexample, the cathode electrode may allow a current to pass more freelyin a first direction than in a second direction (or prevent or impedecurrent in the second direction) with the cathode configured to blockleakage current.

Generally, the portion of the sidewalls to which the IR radiation iscoupled may be selected to allow for reduced overall irradiation powerrequired, while still providing for efficient conversion of heavy holes.For example, as the IR radiation is more efficiently used closer to theanode, the second portion of the at least one sidewall (the portion notirradiated by the IR radiation) is disposed proximate to the cathodeelectrode. In various embodiments, the first portion (the portionirradiated by the IR radiation) corresponds to a non-depleted region.For example, the first portion may cover all or a portion of thenon-depleted region. Also, the second portion (the portion notirradiated by the IR radiation) may correspond to a depletion region.For example, the second portion may cover all or a portion of thedepletion region. It may be noted that in various embodiments, thesecond portion (the portion not irradiated by the IR radiation) mayinclude between 10% and 30% of a total thickness of the detector (e.g.,the distance between first and second surfaces between which thesidewalls extend). As discussed herein, the IR radiation may beoptically coupled into the semiconductor plate using a laser and/or alight guide. For example, the laser may be aimed or directed to provideoptical excitation energy to the desired portion of the sidewalls.Additionally or alternatively, a light guide may be coupled to thesidewalls and configured to provide the IR radiation to the desiredportion of sidewalls.

FIG. 11 provides a schematic view of a radiation detector assembly 600in accordance with various embodiments. The radiation detector assembly600 (or aspects thereof) may be similarly configured to the radiationdetector assembly 100 discussed herein in various respects. As seen inFIG. 11, the radiation detector assembly 600 includes a semiconductorplate 610, a processing unit 620, and an infrared (IR) radiation source630. Generally, the semiconductor plate 610 is configured to produceelectrical signals in response to absorption of ionizing radiation inthe semiconductor plate 610. It may be noted that electrons and holesare generated responsive to absorption of the ionizing radiation by thesemiconductor plate 610 as discussed herein. The IR radiation source 630is configured to provide IR radiation to semiconductor plate 610. Theprocessing unit 620 is operably coupled to the semiconductor plate 610,and is configured to provide IR radiation into the semiconductor plate610 from the IR radiation source 630 via at least a portion of sidewalls616 of the semiconductor plate 610. The IR radiation may be utilized toconvert holes from one valance band to another during the reception ofionizing radiation, via cathode 612, from an object being imaged. Forexample, as discussed herein, the IR radiation includes at least onewavelength selected (e.g., determined by the processing unit 620) from aspectral range including wavelengths to which the semiconductor plate610 is partially transparent and which are configured to excite at leastsome of the holes (e.g., holes generated by the absorption of ionizingradiation by the semiconductor plate 110) from a first group at a firstvalence energy band to a second group of holes a second valence energyband, with the holes of the second group having lower effective massesthan corresponding holes of the first group.

As seen in FIG. 11, the semiconductor plate 610 includes a first surface611, a second surface 613, sidewalls 616, a monolithic cathode electrode612, and pixelated anode electrodes 614. In the depicted embodiment, thecathode electrode 612 is a blocking electrode having a non-symmetricalcurrent response. For example, the cathode electrode 612 is configuredto allow a current to pass more freely in a first direction than in asecond direction (or prevent or impede current in the second direction)with the cathode electrode 612 configured to block leakage current. Itmay be noted that in some embodiments plural cathode electrodes may beemployed instead of a monolithic cathode electrode.

In the illustrated embodiment, the first surface 611 is opposed to thesecond surface 613, with the sidewalls 616 interposed between the firstsurface 611 and the second surface 613, and extending therebetween. Themonolithic cathode electrode 612 is disposed on the first surface 611,and the pixelated anode electrodes 614 are disposed on the secondsurface 613.

As seen in FIG. 11, the IR radiation source 630 in the depictedembodiment is configured to direct IR radiation into the semiconductorplate 610 via a first portion 650 of at least one of the sidewalls 616,and to not direct IR radiation into the semiconductor plate 610 via asecond portion 652 of the at least one of the sidewalls 616. In theillustrated embodiment, the IR radiation source 630 couples IR radiationinto the semiconductor through only the first portion 650. The firstportion 650 and the second portion 652 may be selected to allow forreduced overall irradiation power required, while still providing forefficient conversion of heavy holes. For example, as the IR radiation ismore efficiently used closer to the anode electrodes 614, the secondportion 652 of the at least one sidewall (the portion not irradiated bythe IR radiation) may be disposed proximate to the cathode electrode612. Also, the first portion 650 may be disposed proximate to the anodeelectrodes 614. In various embodiments, the first portion 650 isselected or configured to correspond to a non-depleted region. Forexample, the first portion 650 may cover all or a portion of thenon-depleted region of the semiconductor plate via one or more of thesidewalls 616. Also, the second portion 652 (the portion not irradiatedby the IR radiation) may be selected or configured to correspond to adepletion region. For example, the second portion 652 may cover all or aportion of the depletion region of the semiconductor plate 610 via oneor more of the sidewalls 616. It may be noted that in variousembodiments, the second portion 652 (the portion not irradiated by theIR radiation) may include between 10% and 30% of a total thickness ofthe detector (e.g., the distance between first and second surfacesbetween which the sidewalls extend).

It may be noted that the semiconductor plate 610 may be constructedusing different materials, such as semiconductor materials, includingCadmium Zinc Telluride (CdZnTe), often referred to as CZT, CadmiumTelluride (CdTe), or other semiconductors having split valence energybands. Generally, when radiation (e.g., one or more photons) impacts thepixelated anode electrodes 614, the semiconductor plate 610 generateselectrical signals corresponding to the radiation being absorbed in thevolume of the detector, with the signals provided to the processing unit620 via one or more electronics channels.

In the depicted embodiment, the IR radiation source 630 is configured toprovide IR radiation to the semiconductor plate 610 via the firstportion 650 of at least one of the sidewalls 616. The depicted IRradiation source 630 includes a light source 632 and a light guide 634coupled to the light source 632. The light guide 634 is interposedbetween the light source 632 and the semiconductor plate 610 and guideslight from the light source 632 to the semiconductor plate 610. A singlelight guide 634 is schematically depicted in FIG. 11; however,additional light guides 634 may be employed. For example, one or moresides of the detector 600 may have a light guide 634 associatedtherewith. As discussed herein, the light guide 634 may include one ormore Bragg grating. In various embodiments, the IR radiation source 630may include, for example, a CO₂ laser having emission lines at 9.4 μmand 10.6 μm wavelengths. In various embodiments, the light source 632may be aimed or directed to provide optical excitation energy to thefirst portion 650. Additionally or alternatively, the light guide 634may be coupled to the one or more of the sidewalls 616 and configured toprovide the IR radiation to the first portion 650.

As seen in FIG. 11, the depicted processing unit 620 is operably coupledto the semiconductor plate 610, as well as to the IR radiation source630. The processing unit 620 is configured to, among other things,determine or otherwise select a wavelength and/or intensity of IRradiation to provide to the semiconductor plate 610 to convert holesgenerated by absorbed ionizing radiation from an object to be imagedfrom at least one valence band to at least one other valence band havinglower effective mass (and improved mobility), and to control the IRradiation source 630 to provide the IR radiation at the determinedwavelength (or wavelengths) and/or at the determined intensity.

In various embodiments the processing unit 620 includes processingcircuitry configured to perform one or more tasks, functions, or stepsdiscussed herein (e.g., steps discussed in connection with FIG. 5 and/orFIG. 3). It may be noted that “processing unit” as used herein is notintended to necessarily be limited to a single processor or computer.For example, the processing unit 620 may include multiple processors,ASIC's and/or computers, which may be integrated in a common housing orunit, or which may distributed among various units or housings. It maybe noted that operations performed by the processing unit 620 (e.g.,operations corresponding to process flows or methods discussed herein,or aspects thereof) may be sufficiently complex that the operations maynot be performed by a human being within a reasonable time period.

In the illustrated embodiment, the processing unit 620 includes acontrol module 622 and a memory 624. It may be noted that other types,numbers, or combinations of modules may be employed in alternateembodiments, and/or various aspects of modules described herein may beutilized in connection with different modules additionally oralternatively. For example, the processing unit 620 may include one ormore modules configured to acquire data from the semiconductor plate 610and to reconstruct one or more images using the acquired data.Generally, the various aspects of the processing unit 620 actindividually or cooperatively with other aspects to perform one or moreaspects of the methods, steps, or processes discussed herein.

In the illustrated embodiment, the depicted control module 622 isconfigured to determine one or more wavelengths of IR radiation and/orintensity of IR radiation to provide to the semiconductor plate 610 forimproving the mobility of holes within a valence band. The controlmodule 622 is also configured to control the IR radiation source 630 toprovide the IR radiation to the semiconductor plate 610. Additionally oralternatively, the control module 622 may be configured to determine thelocation(s) at which IR radiation is provided (e.g., determined the sizeand or location of the first portion 650) and/or to control the IRradiation source 630 to provide the radiation at the desired location(s)(e.g., the first portion 650).

The memory 624 may include one or more computer readable storage media.Further, the process flows and/or flowcharts discussed herein (oraspects thereof) may represent one or more sets of instructions that arestored in the memory 624 for direction of operations of the radiationdetection assembly 600.

It should be noted that the particular arrangement of components (e.g.,the number, types, placement, or the like) of the illustratedembodiments may be modified in various alternate embodiments. Forexample, in various embodiments, different numbers of a given module orunit may be employed, a different type or types of a given module orunit may be employed, a number of modules or units (or aspects thereof)may be combined, a given module or unit may be divided into pluralmodules (or sub-modules) or units (or sub-units), one or more aspects ofone or more modules may be shared between modules, a given module orunit may be added, or a given module or unit may be omitted.

As used herein, a structure, limitation, or element that is “configuredto” perform a task or operation is particularly structurally formed,constructed, or adapted in a manner corresponding to the task oroperation. For purposes of clarity and the avoidance of doubt, an objectthat is merely capable of being modified to perform the task oroperation is not “configured to” perform the task or operation as usedherein. Instead, the use of “configured to” as used herein denotesstructural adaptations or characteristics, and denotes structuralrequirements of any structure, limitation, or element that is describedas being “configured to” perform the task or operation.

As used herein, the term “computer,” “processor,” or “module” mayinclude any processor-based or microprocessor-based system includingsystems using microcontrollers, reduced instruction set computers(RISC), application specific integrated circuits (ASICs), logiccircuits, and any other circuit or processor capable of executing thefunctions described herein. The above examples are exemplary only, andare thus not intended to limit in any way the definition and/or meaningof the term “computer,” “processor,” or “module.”

The computer or processor executes a set of instructions that are storedin one or more storage elements, in order to process input data. Thestorage elements may also store data or other information as desired orneeded. The storage element may be in the form of an information sourceor a physical memory element within a processing machine.

The set of instructions may include various commands that instruct thecomputer or processor as a processing machine to perform specificoperations such as the methods and processes of the various embodimentsof the invention. The set of instructions may be in the form of asoftware program. The software may be in various forms such as systemsoftware or application software. Further, the software may be in theform of a collection of separate programs or modules, a program modulewithin a larger program or a portion of a program module. The softwarealso may include modular programming in the form of object-orientedprogramming. The processing of input data by the processing machine maybe in response to operator commands, or in response to results ofprevious processing, or in response to a request made by anotherprocessing machine.

As used herein, the terms “software” and “firmware” may include anycomputer program stored in memory for execution by a computer, includingRAM memory, ROM memory, EPROM memory, EEPROM memory, and non-volatileRAM (NVRAM) memory. The above memory types are exemplary only, and arethus not limiting as to the types of memory usable for storage of acomputer program.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the variousembodiments of the invention without departing from their scope. Whilethe dimensions and types of materials described herein are intended todefine the parameters of the various embodiments of the invention, theembodiments are by no means limiting and are exemplary embodiments. Manyother embodiments will be apparent to those of skill in the art uponreviewing the above description. The scope of the various embodiments ofthe invention should, therefore, be determined with reference to theappended claims, along with the full scope of equivalents to which suchclaims are entitled.

In the appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein.” Moreover, in the following claims, the terms “first,”“second,” and “third,” etc. are used merely as labels, and are notintended to impose numerical requirements on their objects. Further, thelimitations of the following claims are not written inmeans-plus-function format and are not intended to be interpreted basedon 35 U.S.C. § 112(f), unless and until such claim limitations expresslyuse the phrase “means for” followed by a statement of function void offurther structure.

This written description uses examples to disclose the variousembodiments of the invention, including the best mode, and also toenable any person skilled in the art to practice the various embodimentsof the invention, including making and using any devices or systems andperforming any incorporated methods. The patentable scope of the variousembodiments of the invention is defined by the claims, and may includeother examples that occur to those skilled in the art. Such otherexamples are intended to be within the scope of the claims if theexamples have structural elements that do not differ from the literallanguage of the claims, or if the examples include equivalent structuralelements with insubstantial differences from the literal language of theclaims.

What is claimed is:
 1. A method comprising: acquiring detection events with a radiation detector comprising a semiconductor plate, wherein the detector is configured to produce electrical signals in response to absorption of ionizing radiation in the semiconductor plate, wherein electrons and holes are generated responsive to absorption of the ionizing radiation, the holes including groups of holes having different effective masses for corresponding different valence energy bands, wherein the semiconductor plate comprises a first surface, a second surface, and sidewalls, the first surface opposed to the second surface and the sidewalls interposed between the first surface and the second surface, wherein a cathode electrode is disposed on the first surface and pixelated anode electrodes disposed on the second surface; and optically coupling infrared (IR) radiation into a first portion of at least one of the sidewalls of the semiconductor plate of the radiation detector, and not coupling IR radiation into a second portion of the at least one of the sidewalls, the IR radiation having at least one wavelength selected from a spectral range including wavelengths to which the semiconductor plate is partially transparent, the IR radiation having an intensity, wherein the intensity and at least one wavelength are configured to excite at least some of the holes from a first group at a first valence energy band to a second group at a second valence energy band, wherein the holes of the second group have lower effective masses than corresponding holes of the first group.
 2. The method of claim 1, wherein the second portion of the at least one of the sidewalls is disposed proximate to the cathode electrode of the radiation detector.
 3. The method of claim 1, wherein the second portion of the at least one of the sidewalls corresponds to a depletion region.
 4. The method of claim 1, wherein the first portion of the at least one of the sidewalls corresponds to a non-depleted region.
 5. The method of claim 1, wherein the second portion of the at least one of the sidewalls comprises between 10% and 30% of a thickness of the radiation detector.
 6. The method of claim 1, wherein the cathode is a blocking cathode having a non-symmetrical current response.
 7. The method of claim 1, wherein the spectral range includes wavelengths in the range from 3 micrometers (μm) to 16.5 micrometers (μm).
 8. The method of claim 1, wherein optically coupling IR radiation into the semiconductor plate comprises generating wavelengths of 9.4 micrometers (μm) and 10.6 micrometers (μm) with a laser.
 9. The method of claim 1, wherein the intensity of the IR radiation is selected based on at least one of detector temperature, amount of doping in the semiconductor plate, composition of the semiconductor plate, or intensity of the ionizing radiation.
 10. The method of claim 1, wherein the intensity of the IR radiation is less than an intensity required to convert all heavy holes into light holes for the detector.
 11. The method of claim 1, wherein the intensity of the IR radiation is less than 0.066 watt/square centimeter.
 12. A radiation detector comprising: a semiconductor plate, wherein the detector is configured to produce electrical signals in response to absorption of ionizing radiation in the semiconductor plate, wherein electrons and holes are generated responsive to absorption of the ionizing radiation, the holes including groups of holes having different effective masses for corresponding different valence energy bands, wherein the semiconductor plate comprises a first surface, a second surface, sidewalls, a blocking cathode electrode having a non-symmetrical current response, and pixelated anode electrodes, the first surface opposed to the second surface and the sidewalls interposed between the first surface and the second surface, the blocking cathode electrode disposed on the first surface and the pixelated anode electrodes disposed on the second surface; an infrared (IR) radiation source configured to provide IR radiation to the semiconductor plate via at least one sidewall; and at least one processor operably coupled to the semiconductor plate, the at least one processor configured to provide IR radiation into the semiconductor plate from the IR radiation source, the IR radiation having at least one wavelength selected from a spectral range including wavelengths to which the semiconductor plate is partially transparent, the IR radiation having an intensity, wherein the intensity and at least one wavelength are configured to excite at least some of the holes from a first group at a first valence energy band to a second group at a second valence energy band, wherein the holes of the second group have lower effective masses than corresponding holes of the first group.
 13. The radiation detector of claim 12, wherein the IR radiation source is configured to optically couple the IR radiation into a first portion of at least one of the sidewalls, and to not couple IR radiation into a second portion of the at least one of the sidewalls.
 14. The radiation detector of claim 13, wherein the second portion of the at least one of the sidewalls is disposed proximate to the cathode electrode of the radiation detector.
 15. The radiation detector of claim 14, wherein the IR radiation source comprises a light source and light guides coupled to the light source.
 16. The radiation detector of claim 13, wherein the second portion of the at least one of the sidewalls corresponds to a depletion region, and wherein the first portion of the at least one of the sidewalls corresponds to a non-depleted region.
 17. The radiation detector of claim 12, wherein the second portion of the at least one of the sidewalls comprises between 10% and 30% of a thickness of the radiation detector.
 18. The radiation detector of claim 12, wherein the IR radiation source is configured to provide the IR radiation at an intensity less than an intensity required to convert all heavy holes into light holes for the detector.
 19. A tangible and non-transitory computer readable medium comprising one or more software modules configured to direct one or more processors to: acquire detection events via a radiation detector comprising a semiconductor plate, wherein the detector is configured to produce electrical signals in response to absorption of ionizing radiation in the semiconductor plate, wherein electrons and holes are generated responsive to absorption of the ionizing radiation, the holes including groups of holes having different effective masses for corresponding different valence energy bands, wherein the semiconductor plate comprises a first surface, a second surface, and sidewalls, the first surface opposed to the second surface and the sidewalls interposed between the first surface and the second surface, wherein a blocking cathode electrode is disposed on the first surface and pixelated anode electrodes disposed on the second surface; and optically couple infrared (IR) radiation into a first portion of at least one of the sidewalls of the semiconductor plate of the radiation detector, and not couple IR radiation into a second portion of the at least one of the sidewalls, the IR radiation having at least one wavelength selected from a spectral range including wavelengths to which the semiconductor plate is partially transparent, the IR radiation having an intensity, wherein the intensity and at least one wavelength are configured to excite at least some of the holes from a first group at a first valence energy band to a second group at a second valence energy band, wherein the holes of the second group have lower effective masses than corresponding holes of the first group.
 20. The tangible and non-transitory computer readable medium of claim 19, wherein the intensity of the IR radiation is less than an intensity required to convert all heavy holes into light holes for the detector. 